diff --git a/src/libraries/System.Private.CoreLib/src/System/Half.cs b/src/libraries/System.Private.CoreLib/src/System/Half.cs
index 1f73acf698f795..f1e653e60e1fa5 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Half.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Half.cs
@@ -1536,7 +1536,7 @@ public static int ILogB(Half x)
                 }
 
                 Debug.Assert(IsSubnormal(x));
-                return MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
+                return MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - BiasedExponentLength);
             }
 
             return x.Exponent;
diff --git a/src/libraries/System.Private.CoreLib/src/System/Math.cs b/src/libraries/System.Private.CoreLib/src/System/Math.cs
index a6549b8807ae0c..b69ad824a68c76 100644
--- a/src/libraries/System.Private.CoreLib/src/System/Math.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/Math.cs
@@ -895,7 +895,7 @@ public static int ILogB(double x)
                 }
 
                 Debug.Assert(double.IsSubnormal(x));
-                return double.MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - double.BiasedExponentLength);
+                return double.MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - double.BiasedExponentLength);
             }
 
             return x.Exponent;
diff --git a/src/libraries/System.Private.CoreLib/src/System/MathF.cs b/src/libraries/System.Private.CoreLib/src/System/MathF.cs
index 834b4f5e9606ca..01f0d31f701694 100644
--- a/src/libraries/System.Private.CoreLib/src/System/MathF.cs
+++ b/src/libraries/System.Private.CoreLib/src/System/MathF.cs
@@ -210,7 +210,7 @@ public static int ILogB(float x)
                 }
 
                 Debug.Assert(float.IsSubnormal(x));
-                return float.MinExponent - (BitOperations.TrailingZeroCount(x.TrailingSignificand) - float.BiasedExponentLength);
+                return float.MinExponent - (BitOperations.LeadingZeroCount(x.TrailingSignificand) - float.BiasedExponentLength);
             }
 
             return x.Exponent;
diff --git a/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Math.cs b/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Math.cs
index 2550b706b41063..316650591fd64f 100644
--- a/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Math.cs
+++ b/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/Math.cs
@@ -2287,6 +2287,7 @@ public static void FusedMultiplyAdd(double x, double y, double z, double expecte
         [InlineData( 0.561760,                -1)]
         [InlineData( 0.774152,                -1)]
         [InlineData( -0.678764,               -1)]
+        [InlineData( 1e-308,               -1024)]
         public static void ILogB(double value, int expectedResult)
         {
             Assert.Equal(expectedResult, Math.ILogB(value));
diff --git a/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/MathF.cs b/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/MathF.cs
index 1363a2b407b016..e5d2b224734b13 100644
--- a/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/MathF.cs
+++ b/src/libraries/System.Runtime/tests/System.Runtime.Extensions.Tests/System/MathF.cs
@@ -742,6 +742,7 @@ public static void IEEERemainder()
         [InlineData(0.561760f, -1)]
         [InlineData(0.774152f, -1)]
         [InlineData(-0.678764f, -1)]
+        [InlineData(1e-44f, -147)]
         public static void ILogB(float value, int expectedResult)
         {
             Assert.Equal(expectedResult, MathF.ILogB(value));