diff --git a/CHANGELOG.md b/CHANGELOG.md
index 8cc620f6..e56df48e 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,7 @@
 - _...Add new stuff here..._
 
 ### 🐞 Bug fixes
+- The `index-of`, `length`, and `slice` expression operators count a UTF-16 surrogate pair as a single character. ([#779](https://github.com/maplibre/maplibre-style-spec/pull/779))
 - _...Add new stuff here..._
 
 ## 20.3.0
diff --git a/src/expression/definitions/index_of.ts b/src/expression/definitions/index_of.ts
index 844a4573..a8202ad0 100644
--- a/src/expression/definitions/index_of.ts
+++ b/src/expression/definitions/index_of.ts
@@ -60,16 +60,24 @@ class IndexOf implements Expression {
             throw new RuntimeError(`Expected first argument to be of type boolean, string, number or null, but found ${toString(typeOf(needle))} instead.`);
         }
 
-        if (!isValidNativeType(haystack, ['string', 'array'])) {
-            throw new RuntimeError(`Expected second argument to be of type array or string, but found ${toString(typeOf(haystack))} instead.`);
+        let fromIndex;
+        if (this.fromIndex) {
+            fromIndex = (this.fromIndex.evaluate(ctx) as number);
         }
 
-        if (this.fromIndex) {
-            const fromIndex = (this.fromIndex.evaluate(ctx) as number);
+        if (isValidNativeType(haystack, ['string'])) {
+            const rawIndex = haystack.indexOf(needle, fromIndex);
+            if (rawIndex === -1) {
+                return -1;
+            } else {
+                // The index may be affected by surrogate pairs, so get the length of the preceding substring.
+                return [...haystack.slice(0, rawIndex)].length;
+            }
+        } else if (isValidNativeType(haystack, ['array'])) {
             return haystack.indexOf(needle, fromIndex);
+        } else {
+            throw new RuntimeError(`Expected second argument to be of type array or string, but found ${toString(typeOf(haystack))} instead.`);
         }
-
-        return haystack.indexOf(needle);
     }
 
     eachChild(fn: (_: Expression) => void) {
diff --git a/src/expression/definitions/length.ts b/src/expression/definitions/length.ts
index 2579bd05..ba5ec56c 100644
--- a/src/expression/definitions/length.ts
+++ b/src/expression/definitions/length.ts
@@ -33,7 +33,8 @@ class Length implements Expression {
     evaluate(ctx: EvaluationContext) {
         const input = this.input.evaluate(ctx);
         if (typeof input === 'string') {
-            return input.length;
+            // The length may be affected by surrogate pairs.
+            return [...input].length;
         } else if (Array.isArray(input)) {
             return input.length;
         } else {
diff --git a/src/expression/definitions/slice.ts b/src/expression/definitions/slice.ts
index d550941d..94f92371 100644
--- a/src/expression/definitions/slice.ts
+++ b/src/expression/definitions/slice.ts
@@ -56,16 +56,19 @@ class Slice implements Expression {
         const input = (this.input.evaluate(ctx) as any);
         const beginIndex = (this.beginIndex.evaluate(ctx) as number);
 
-        if (!isValidNativeType(input, ['string', 'array'])) {
-            throw new RuntimeError(`Expected first argument to be of type array or string, but found ${toString(typeOf(input))} instead.`);
+        let endIndex;
+        if (this.endIndex) {
+            endIndex = (this.endIndex.evaluate(ctx) as number);
         }
 
-        if (this.endIndex) {
-            const endIndex = (this.endIndex.evaluate(ctx) as number);
+        if (isValidNativeType(input, ['string'])) {
+            // Indices may be affected by surrogate pairs.
+            return [...input].slice(beginIndex, endIndex).join('');
+        } else if (isValidNativeType(input, ['array'])) {
             return input.slice(beginIndex, endIndex);
+        } else {
+            throw new RuntimeError(`Expected first argument to be of type array or string, but found ${toString(typeOf(input))} instead.`);
         }
-
-        return input.slice(beginIndex);
     }
 
     eachChild(fn: (_: Expression) => void) {
diff --git a/src/expression/expression.test.ts b/src/expression/expression.test.ts
index d45d9bbc..605d2ab6 100644
--- a/src/expression/expression.test.ts
+++ b/src/expression/expression.test.ts
@@ -388,3 +388,244 @@ describe('Distance expression', () => {
         });
     });
 });
+
+describe('index-of expression', () => {
+    test('requires a needle', () => {
+        const response = createExpression(['index-of']);
+        expect(response.result).toBe('error');
+    });
+    test('requires a haystack', () => {
+        const response = createExpression(['index-of', 'a']);
+        expect(response.result).toBe('error');
+    });
+    test('rejects a fourth argument', () => {
+        const response = createExpression(['index-of', 'a', 'abc', 1, 8]);
+        expect(response.result).toBe('error');
+    });
+    test('requires a primitive as the needle', () => {
+        const response = createExpression(['index-of', ['literal', ['a']], ['a', 'b', 'c']]);
+        expect(response.result).toBe('error');
+    });
+    test('requires a string or array as the haystack', () => {
+        const response = createExpression(['index-of', 't', true]);
+        expect(response.result).toBe('error');
+    });
+    test('finds an empty substring in an empty string', () => {
+        const response = createExpression(['index-of', '', '']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(0);
+    });
+    test('finds an empty substring in a non-empty string', () => {
+        const response = createExpression(['index-of', '', 'abc']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(0);
+    });
+    test('cannot find a non-empty substring in an empty string', () => {
+        const response = createExpression(['index-of', 'abc', '']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(-1);
+    });
+    test('finds a non-empty substring in a non-empty string', () => {
+        const response = createExpression(['index-of', 'b', 'abc']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(1);
+    });
+    test('only finds the first occurrence in a string', () => {
+        const response = createExpression(['index-of', 'b', 'abbc']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(1);
+    });
+    test('starts looking for the substring at a positive start index', () => {
+        const response = createExpression(['index-of', 'a', 'abc', 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(-1);
+    });
+    test('starts looking for the substring at a negative start index', () => {
+        const response = createExpression(['index-of', 'c', 'abc', -1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(2);
+    });
+    test('counts a non-ASCII character as a single character', () => {
+        const response = createExpression(['index-of', '镇', '市镇']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(1);
+    });
+    test('counts a surrogate pair as a single character', () => {
+        const response = createExpression(['index-of', '市镇', '丐𦨭市镇']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(2);
+    });
+    test('cannot find an element in an empty array', () => {
+        const response = createExpression(['index-of', 1, ['literal', []]]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(-1);
+    });
+    test('finds an element in a non-empty array', () => {
+        const response = createExpression(['index-of', 2, ['literal', [1, 2, 3]]]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(1);
+    });
+    test('only finds the first occurrence in an array', () => {
+        const response = createExpression(['index-of', 2, ['literal', [1, 2, 2, 3]]]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(1);
+    });
+    test('starts looking for the element at a positive start index', () => {
+        const response = createExpression(['index-of', 1, ['literal', [1, 2, 3]], 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(-1);
+    });
+    test('starts looking for the element at a negative start index', () => {
+        const response = createExpression(['index-of', 3, ['literal', [1, 2, 3]], -1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(2);
+    });
+});
+
+describe('length expression', () => {
+    test('requires an argument', () => {
+        const response = createExpression(['length']);
+        expect(response.result).toBe('error');
+    });
+    test('requires a string or array as the argument', () => {
+        const response = createExpression(['length', true]);
+        expect(response.result).toBe('error');
+    });
+    test('rejects a second argument', () => {
+        const response = createExpression(['length', 'abc', 'def']);
+        expect(response.result).toBe('error');
+    });
+    test('measures an empty string', () => {
+        const response = createExpression(['length', '']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(0);
+    });
+    test('measures a non-empty string', () => {
+        const response = createExpression(['length', 'abc']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(3);
+    });
+    test('counts a non-ASCII character as a single character', () => {
+        const response = createExpression(['length', '市镇']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(2);
+    });
+    test('counts a surrogate pair as a single character', () => {
+        const response = createExpression(['length', '丐𦨭市镇']);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(4);
+    });
+    test('measures an empty array', () => {
+        const response = createExpression(['length', ['literal', []]]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(0);
+    });
+    test('measures a non-empty array', () => {
+        const response = createExpression(['length', ['literal', [1, 2, 3]]]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe(3);
+    });
+});
+
+describe('slice expression', () => {
+    test('requires an input argument', () => {
+        const response = createExpression(['slice']);
+        expect(response.result).toBe('error');
+    });
+    test('requires a start index argument', () => {
+        const response = createExpression(['slice', 'abc']);
+        expect(response.result).toBe('error');
+    });
+    test('rejects a fourth argument', () => {
+        const response = createExpression(['slice', 'abc', 0, 1, 8]);
+        expect(response.result).toBe('error');
+    });
+    test('requires a string or array as the input argument', () => {
+        const response = createExpression(['slice', true, 0]);
+        expect(response.result).toBe('error');
+    });
+    test('requires a number as the start index argument', () => {
+        const response = createExpression(['slice', 'abc', true]);
+        expect(response.result).toBe('error');
+    });
+    test('slices an empty string', () => {
+        const response = createExpression(['slice', '', 0]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('');
+    });
+    test('slices a string starting at the beginning', () => {
+        const response = createExpression(['slice', 'abc', 0]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('abc');
+    });
+    test('slices a string starting at the middle', () => {
+        const response = createExpression(['slice', 'abc', 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('bc');
+    });
+    test('slices a string starting at the end', () => {
+        const response = createExpression(['slice', 'abc', 3]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('');
+    });
+    test('slices a string backwards from the end', () => {
+        const response = createExpression(['slice', 'abc', -2]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('bc');
+    });
+    test('slices a string by a zero-length range', () => {
+        const response = createExpression(['slice', 'abc', 1, 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('');
+    });
+    test('slices a string by a negative-length range', () => {
+        const response = createExpression(['slice', 'abc', 2, 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('');
+    });
+    test('avoids splitting a non-ASCII character', () => {
+        const response = createExpression(['slice', '市镇', 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('镇');
+    });
+    test('avoids splitting a surrogate pair', () => {
+        const response = createExpression(['slice', '丐𦨭市镇', 2]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toBe('市镇');
+    });
+    test('slices an empty array', () => {
+        const response = createExpression(['slice', ['literal', []], 0]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([]);
+    });
+    test('slices an array starting at the beginning', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], 0]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([1, 2, 3]);
+    });
+    test('slices an array starting at the middle', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([2, 3]);
+    });
+    test('slices an array starting at the end', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], 3]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([]);
+    });
+    test('slices an array backwards from the end', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], -2]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([2, 3]);
+    });
+    test('slices an array by a zero-length range', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], 1, 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([]);
+    });
+    test('slices an array by a negative-length range', () => {
+        const response = createExpression(['slice', ['literal', [1, 2, 3]], 2, 1]);
+        expect(response.result).toBe('success');
+        expect((response.value as StyleExpression)?.evaluate({zoom: 20})).toEqual([]);
+    });
+});
diff --git a/src/reference/v8.json b/src/reference/v8.json
index 889de1b7..c9d35dc1 100644
--- a/src/reference/v8.json
+++ b/src/reference/v8.json
@@ -2826,7 +2826,7 @@
         }
       },
       "index-of": {
-        "doc": "Returns the first position at which an item can be found in an array or a substring can be found in a string, or `-1` if the input cannot be found. Accepts an optional index from where to begin the search.",
+        "doc": "Returns the first position at which an item can be found in an array or a substring can be found in a string, or `-1` if the input cannot be found. Accepts an optional index from where to begin the search. In a string, a UTF-16 surrogate pair counts as a single position.",
         "example": {
           "syntax": {
             "method": ["value", "value", "number?"],
@@ -2844,7 +2844,7 @@
         }
       },
       "slice": {
-        "doc": "Returns an item from an array or a substring from a string from a specified start index, or between a start index and an end index if set. The return value is inclusive of the start index but not of the end index.",
+        "doc": "Returns an item from an array or a substring from a string from a specified start index, or between a start index and an end index if set. The return value is inclusive of the start index but not of the end index. In a string, a UTF-16 surrogate pair counts as a single position.",
         "example": {
           "syntax": {
             "method": ["value", "number", "number?"],
@@ -3380,7 +3380,7 @@
         }
       },
       "length": {
-        "doc": "Gets the length of an array or string.",
+        "doc": "Gets the length of an array or string. In a string, a UTF-16 surrogate pair counts as a single position.",
         "example": {
           "syntax": {
             "method": ["array"],