diff --git a/src/Data/Validation.hs b/src/Data/Validation.hs
index 7e19153..c1f6338 100644
--- a/src/Data/Validation.hs
+++ b/src/Data/Validation.hs
@@ -59,7 +59,6 @@ import Data.Functor(Functor(fmap))
 import Data.Functor.Alt(Alt((<!>)))
 import Data.Functor.Apply(Apply((<.>)))
 import Data.List.NonEmpty (NonEmpty)
-import Data.Monoid(Monoid(mappend, mempty))
 import Data.Ord(Ord)
 import Data.Semigroup(Semigroup((<>)))
 import Data.Traversable(Traversable(traverse))
@@ -164,10 +163,10 @@ appValidation ::
   -> Validation err a
 appValidation m (Failure e1) (Failure e2) =
   Failure (e1 `m` e2)
-appValidation _ (Failure _) (Success a2) =
-  Success a2
-appValidation _ (Success a1) (Failure _) =
-  Success a1
+appValidation _ (Failure e1) (Success _) =
+  Failure e1
+appValidation _ (Success _) (Failure e2) =
+  Failure e2
 appValidation _ (Success a1) (Success _) =
   Success a1
 {-# INLINE appValidation #-}
@@ -177,14 +176,6 @@ instance Semigroup e => Semigroup (Validation e a) where
     appValidation (<>)
   {-# INLINE (<>) #-}
 
-instance Monoid e => Monoid (Validation e a) where
-  mappend =
-    appValidation mappend
-  {-# INLINE mappend #-}
-  mempty =
-    Failure mempty
-  {-# INLINE mempty #-}
-
 instance Swapped Validation where
   swapped =
     iso
diff --git a/test/hedgehog_tests.hs b/test/hedgehog_tests.hs
index 3a4e6a8..60553ee 100644
--- a/test/hedgehog_tests.hs
+++ b/test/hedgehog_tests.hs
@@ -18,9 +18,6 @@ main = do
 
   result <- checkParallel $ Group "Validation"
     [ ("prop_semigroup", prop_semigroup)
-    , ("prop_monoid_assoc", prop_monoid_assoc)
-    , ("prop_monoid_left_id", prop_monoid_left_id)
-    , ("prop_monoid_right_id", prop_monoid_right_id)
     ]
 
   unless result $
@@ -44,19 +41,3 @@ mkAssoc f =
 
 prop_semigroup :: Property
 prop_semigroup = mkAssoc (<>)
-
-prop_monoid_assoc :: Property
-prop_monoid_assoc = mkAssoc mappend
-
-prop_monoid_left_id :: Property
-prop_monoid_left_id =
-  property $ do
-    x <- forAll testGen
-    (mempty `mappend` x) === x
-
-prop_monoid_right_id :: Property
-prop_monoid_right_id =
-  property $ do
-    x <- forAll testGen
-    (x `mappend` mempty) === x
-
diff --git a/test/hunit_tests.hs b/test/hunit_tests.hs
index 2e2340f..67b7d23 100644
--- a/test/hunit_tests.hs
+++ b/test/hunit_tests.hs
@@ -9,6 +9,7 @@ import Control.Lens ((#))
 import Control.Monad (when)
 import Data.Foldable (length)
 import Data.Proxy (Proxy (Proxy))
+import Data.Semigroup(Semigroup((<>)))
 import Data.Validation (Validation (Success, Failure), Validate, _Failure, _Success, ensure,
                         orElse, validate, validation, validationNel)
 import System.Exit (exitFailure)
@@ -111,6 +112,38 @@ testValidateNothing =
       option = Nothing :: Maybe Int
   in  TestCase (assertEqual "testValidateFalse" subject expected)
 
+testMappendNY :: Test
+testMappendNY =
+  let v1 = Failure [three]
+      v2 = Success seven
+      subject = v1 <> v2
+      expected = Failure [three]
+  in  TestCase (assertEqual "Failure <> Success" subject expected)
+
+testMappendYN :: Test
+testMappendYN =
+  let v1 = Success three
+      v2 = Failure [seven]
+      subject = v1 <> v2
+      expected = Failure [seven]
+  in  TestCase (assertEqual "Success <> Failure" subject expected)
+
+testMappendYY :: Test
+testMappendYY =
+  let v1 = Success three
+      v2 = Success seven
+      subject = v1 <> v2 :: Validation [Int] Int
+      expected = Success three
+  in  TestCase (assertEqual "Success <> Success" subject expected)
+
+testMappendNN :: Test
+testMappendNN =
+  let v1 = Failure [three]
+      v2 = Failure [seven]
+      subject = v1 <> v2 :: Validation [Int] Int
+      expected = Failure [three, seven]
+  in  TestCase (assertEqual "Failure <> Failure" subject expected)
+
 tests :: Test
 tests =
   let eitherP :: Proxy Either
@@ -123,7 +156,7 @@ tests =
         , testEnsureLeftJust
         , testEnsureRightNothing
         , testEnsureRightJust
-        , testEnsureRightJust' 
+        , testEnsureRightJust'
         , testOrElseLeft
         , testOrElseRight
         ]
@@ -138,6 +171,10 @@ tests =
   , testValidateNothing
   , testValidateJust
   , testValidateJust'
+  , testMappendNY
+  , testMappendYY
+  , testMappendNN
+  , testMappendYN
   ] ++ eithers ++ validations
   where