diff --git a/package.json b/package.json
index d8febdc..c6fe9cd 100644
--- a/package.json
+++ b/package.json
@@ -3,7 +3,7 @@
   "engines": {
     "node": ">=16"
   },
-  "version": "5.0.0",
+  "version": "5.1.0",
   "type": "module",
   "description": "A GLSL ES 1.0 and 3.0 parser and preprocessor that can preserve whitespace and comments",
   "scripts": {
diff --git a/src/preprocessor/preprocessor.test.ts b/src/preprocessor/preprocessor.test.ts
index dec17e5..8e74dbb 100644
--- a/src/preprocessor/preprocessor.test.ts
+++ b/src/preprocessor/preprocessor.test.ts
@@ -333,6 +333,27 @@ foo`;
 foo`);
 });
 
+test(`function macro where source variable is same as macro argument`, () => {
+  const program = `
+#define FN(x, y) x + y
+FN(y, x);
+FN(y.y, x.x);
+FN(yy, xx);
+`;
+
+  const ast = parse(program);
+  preprocessAst(ast);
+
+  // Ensure that if the argument passed to the fn FN(X) has the
+  // same name as the macro definition #define FN(X), it doesn't get expanded
+  // https://github.com/ShaderFrog/glsl-parser/issues/31
+  expect(generate(ast)).toBe(`
+y + x;
+y.y + x.x;
+yy + xx;
+`);
+});
+
 test("macro that isn't macro function call call is expanded", () => {
   const program = `
 #define foo () yes expand
diff --git a/src/preprocessor/preprocessor.ts b/src/preprocessor/preprocessor.ts
index 3015e58..0d1714d 100644
--- a/src/preprocessor/preprocessor.ts
+++ b/src/preprocessor/preprocessor.ts
@@ -198,17 +198,31 @@ const expandFunctionMacro = (
       throw new Error(`'${macroName}': Not enough arguments for macro`);
     }
 
+    // Collect the macro identifiers and build a replacement map from those to
+    // the user defined replacements
+    const argIdentifiers = macroArgs.map(
+      (a) => (a as PreprocessorIdentifierNode).identifier
+    );
+    const argKeys = argIdentifiers.reduce<Record<string, string>>(
+      (acc, identifier, index) => ({
+        ...acc,
+        [identifier]: args[index].trim(),
+      }),
+      {}
+    );
+
     const replacedBody = tokenPaste(
-      macroArgs.reduce(
-        (replaced, macroArg, index) =>
-          replaced.replace(
-            new RegExp(
-              `\\b${(macroArg as PreprocessorIdentifierNode).identifier}\\b`,
-              'g'
-            ),
-            args[index].trim()
-          ),
-        macro.body
+      macro.body.replace(
+        // Replace all instances of macro arguments in the macro definition
+        // (the arg separated by word boundaries) with its user defined
+        // replacement. This one-pass strategy ensures that we won't clobber
+        // previous replacements when the user supplied args have the same names
+        // as the macro arguments
+        new RegExp(
+          '(' + argIdentifiers.map((a) => `\\b${a}\\b`).join(`|`) + ')',
+          'g'
+        ),
+        (match) => (match in argKeys ? argKeys[match] : match)
       )
     );