diff --git a/lib/player.js b/lib/player.js
index 2f8a4b25..3fdb5a88 100644
--- a/lib/player.js
+++ b/lib/player.js
@@ -516,9 +516,18 @@ Player.prototype.groupSetVolume = function (volumeLevel, callback) {
   var actions = [];
   var count = 0;
   var groupVolume = this.groupState.volume;
-  var deltaVolume = volumeLevel - groupVolume;
-  var newVolume;
 
+  var deltaVolume;
+  // If prefixed with + or -
+  if (/^[+-]/.test(volumeLevel)) {
+    deltaVolume = parseInt(volumeLevel);
+    volumeLevel = groupVolume + parseInt(volumeLevel);
+  } else {
+    volumeLevel = parseInt(volumeLevel);
+    deltaVolume = volumeLevel - groupVolume;
+  }
+
+  var newVolume;
   for (var uuid in this.discovery.players) {
     var player = this.discovery.players[uuid];
     if (player.coordinator.uuid != this.uuid) continue;