diff --git a/libacc/acc.cpp b/libacc/acc.cpp
index 6c03657..475e308 100644
--- a/libacc/acc.cpp
+++ b/libacc/acc.cpp
@@ -3821,8 +3821,20 @@
             inp();
         }
         String value;
+        bool appendToValue = true;
         while (ch != '\n' && ch != EOF) {
-            value.append(ch);
+            // Check for '//' comments.
+            if (appendToValue && ch == '/') {
+                inp();
+                if (ch == '/') {
+                    appendToValue = false;
+                } else {
+                    value.append('/');
+                }
+            }
+            if (appendToValue && ch != EOF) {
+                value.append(ch);
+            }
             inp();
         }
         char* pDefn = (char*)mGlobalArena.alloc(value.len() + 1);
diff --git a/libacc/tests/data/defines.c b/libacc/tests/data/defines.c
index 2a6fdf7..840917d 100644
--- a/libacc/tests/data/defines.c
+++ b/libacc/tests/data/defines.c
@@ -1,7 +1,8 @@
 // Simple tests of the C preprocessor
 
-#define A (1 + 2)
+#define A (4 / 2)
+#define B 1 // This is a comment. With a / in it.
 
 int main() {
-    return A;
+    return A + B;
 }