diff --git a/src/turbo-stream.spec.ts b/src/turbo-stream.spec.ts
index 78e0241..a60a827 100644
--- a/src/turbo-stream.spec.ts
+++ b/src/turbo-stream.spec.ts
@@ -120,6 +120,15 @@ test("should encode and decode Map", async () => {
   expect(output).toEqual(input);
 });
 
+test("should preserve Map entry order", async () => {
+  const input = new Map([
+    ["foo", "bar"],
+    ["baz", "qux"],
+  ]);
+  const output = await quickDecode(encode(input));
+  expect([...output as Map<unknown, unknown>]).toEqual([...input])
+})
+
 test("should encode and decode empty Map", async () => {
   const input = new Map();
   const output = await quickDecode(encode(input));
@@ -132,6 +141,12 @@ test("should encode and decode Set", async () => {
   expect(output).toEqual(input);
 });
 
+test("should preserve Set entry order", async () => {
+  const input = new Set(["foo", "bar"]);
+  const output = await quickDecode(encode(input));
+  expect([...output as Set<unknown>]).toEqual([...input]);
+})
+
 test("should encode and decode empty Set", async () => {
   const input = new Set();
   const output = await quickDecode(encode(input));
diff --git a/src/unflatten.ts b/src/unflatten.ts
index 5dbb7b2..ca41de9 100644
--- a/src/unflatten.ts
+++ b/src/unflatten.ts
@@ -117,7 +117,11 @@ function hydrate(this: ThisDecode, index: number): any {
           case TYPE_SET:
             const newSet = new Set();
             hydrated[index] = newSet;
-            for (let i = 1; i < value.length; i++)
+            // Going through the entries from back to front, because the postRun
+            // callbacks run in reverse (from last item to first)
+            // This results in a Set with its item order mirrored if we visit the
+            // first item first.
+            for (let i = value.length - 1; i >= 1; i --)
               stack.push([
                 value[i],
                 (v) => {
@@ -129,7 +133,11 @@ function hydrate(this: ThisDecode, index: number): any {
           case TYPE_MAP:
             const map = new Map();
             hydrated[index] = map;
-            for (let i = 1; i < value.length; i += 2) {
+            // Going through the entries from back to front, because the postRun
+            // callbacks run in reverse (from last item to first)
+            // This results in a Map with its item order mirrored if we visit the
+            // first item first.
+            for (let i = value.length - 2; i >= 1; i -= 2) {
               const r: any[] = [];
               stack.push([
                 value[i + 1],