diff --git a/src/main/java/org/apache/commons/text/TextStringBuilder.java b/src/main/java/org/apache/commons/text/TextStringBuilder.java
index a3997bff87..64baa303a2 100644
--- a/src/main/java/org/apache/commons/text/TextStringBuilder.java
+++ b/src/main/java/org/apache/commons/text/TextStringBuilder.java
@@ -1908,8 +1908,12 @@ public boolean equalsIgnoreCase(final TextStringBuilder other) {
         for (int i = size - 1; i >= 0; i--) {
             final char c1 = thisBuf[i];
             final char c2 = otherBuf[i];
-            if (c1 != c2 && Character.toUpperCase(c1) != Character.toUpperCase(c2)) {
-                return false;
+            if (c1 != c2) {
+                final char u1 = Character.toUpperCase(c1);
+                final char u2 = Character.toUpperCase(c2);
+                if (u1 != u2 && Character.toLowerCase(u1) != Character.toLowerCase(u2)) {
+                    return false;
+                }
             }
         }
         return true;
diff --git a/src/test/java/org/apache/commons/text/TextStringBuilderTest.java b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
index 3aafdb0d19..e2c74cbcf7 100644
--- a/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
+++ b/src/test/java/org/apache/commons/text/TextStringBuilderTest.java
@@ -974,6 +974,16 @@ void testEqualsIgnoreCase() {
         sb2.set("aBc");
         assertTrue(sb1.equalsIgnoreCase(sb2));
 
+        // characters that only fold together via toLowerCase, matching String.equalsIgnoreCase
+        sb1.set("\u004B"); // LATIN CAPITAL LETTER K
+        sb2.set("\u212A"); // KELVIN SIGN, lower-cases to 'k'
+        assertTrue(sb1.equalsIgnoreCase(sb2));
+        assertTrue(sb2.equalsIgnoreCase(sb1));
+
+        sb1.set("\u00E5"); // LATIN SMALL LETTER A WITH RING ABOVE
+        sb2.set("\u212B"); // ANGSTROM SIGN, lower-cases to the same
+        assertTrue(sb1.equalsIgnoreCase(sb2));
+
         final Locale turkish = Locale.forLanguageTag("tr");
         assertTrue(new TextStringBuilder("title").equalsIgnoreCase(new TextStringBuilder("title".toLowerCase(turkish))));
         assertTrue(new TextStringBuilder("title").equalsIgnoreCase(new TextStringBuilder("TITLE".toLowerCase(turkish))));