diff --git a/Data/PQueue/Prio/Max.hs b/Data/PQueue/Prio/Max.hs
--- a/Data/PQueue/Prio/Max.hs
+++ b/Data/PQueue/Prio/Max.hs
@@ -143,6 +143,11 @@
 second' :: (b -> c) -> (a, b) -> (a, c)
 second' f (a, b) = (a, f b)
 
+instance Ord k => Monoid (MaxPQueue k a) where
+  mempty = empty
+  mappend = union
+  mconcat = unions
+
 instance (Ord k, Show k, Show a) => Show (MaxPQueue k a) where
   showsPrec p xs = showParen (p > 10) $
     showString "fromDescList " . shows (toDescList xs)
diff --git a/pqueue.cabal b/pqueue.cabal
--- a/pqueue.cabal
+++ b/pqueue.cabal
@@ -1,5 +1,5 @@
 Name:               pqueue
-Version:            1.3.0
+Version:            1.3.1
 Category:           Data Structures
 Author:             Louis Wasserman
 License:            BSD3