diff --git a/src/Terminal.ts b/src/Terminal.ts
index e887146287..c9bc98ffe5 100644
--- a/src/Terminal.ts
+++ b/src/Terminal.ts
@@ -985,7 +985,7 @@ export class Terminal extends EventEmitter implements ITerminal, IDisposable, II
           : 65;
           break;
         case 'wheel':
-          button = (<WheelEvent>ev).wheelDeltaY > 0
+          button = (<WheelEvent>ev).deltaY < 0
             ? 64
           : 65;
           break;