diff --git a/src/Algebra/LabelledGraph.agda b/src/Algebra/LabelledGraph.agda
index cfd7b05..f82d669 100644
--- a/src/Algebra/LabelledGraph.agda
+++ b/src/Algebra/LabelledGraph.agda
@@ -29,8 +29,9 @@ data _≡_ {A D eq} {d : Dioid D eq} : (x y : LabelledGraph d A) -> Set where
     transitivity : ∀ {x y z : LabelledGraph d A} -> x ≡ y -> y ≡ z -> x ≡ z
 
     -- Congruence
-    left-congruence  : ∀ {x y z : LabelledGraph d A} {r : D} -> x ≡ y -> x [ r ]> z ≡ y [ r ]> z
-    right-congruence : ∀ {x y z : LabelledGraph d A} {r : D} -> x ≡ y -> z [ r ]> x ≡ z [ r ]> y
+    left-congruence  : ∀ {x y z : LabelledGraph d A} {r : D} -> x ≡ y  -> x [ r ]> z ≡ y [ r ]> z
+    right-congruence : ∀ {x y z : LabelledGraph d A} {r : D} -> x ≡ y  -> z [ r ]> x ≡ z [ r ]> y
+    dioid-congruence : ∀ {x y : LabelledGraph d A} {r s : D} -> eq r s -> x [ r ]> y ≡ x [ s ]> y
 
     -- Axioms
     zero-commutativity  : ∀ {x y : LabelledGraph d A} -> x + y ≡ y + x
diff --git a/src/Algebra/LabelledGraph/Theorems.agda b/src/Algebra/LabelledGraph/Theorems.agda
index aebfe67..fbb7b96 100644
--- a/src/Algebra/LabelledGraph/Theorems.agda
+++ b/src/Algebra/LabelledGraph/Theorems.agda
@@ -100,6 +100,7 @@ to-graph (v x) = Graph.v x
 to-graph (x [ Bool.false ]> y) = to-graph x Graph.+ to-graph y
 to-graph (x [ Bool.true  ]> y) = to-graph x Graph.* to-graph y
 
+
 labelled-graph-laws : ∀ {A} {x y : LabelledGraph Bool.bool-dioid A} -> x ≡ y -> (to-graph x) Graph.≡ (to-graph y)
 labelled-graph-laws reflexivity = Graph.reflexivity
 labelled-graph-laws (symmetry eq) = Graph.symmetry (labelled-graph-laws eq)
@@ -111,6 +112,13 @@ labelled-graph-laws (left-congruence {r = r} eq) with r
 labelled-graph-laws (right-congruence {r = r} eq) with r
 ... | Bool.true = Graph.*right-congruence (labelled-graph-laws eq)
 ... | Bool.false = Graph.+right-congruence (labelled-graph-laws eq)
+labelled-graph-laws (dioid-congruence eq) = dioid-recur eq
+  where
+    dioid-recur : ∀ {A} {x y : LabelledGraph Bool.bool-dioid A} {r s : Bool.Bool}
+               -> r Bool.≡ s -> (to-graph (x [ r ]> y)) Graph.≡ (to-graph (x [ s ]> y))
+    dioid-recur Bool.reflexivity = Graph.reflexivity
+    dioid-recur (Bool.symmetry eq) = Graph.symmetry (dioid-recur eq)
+    dioid-recur (Bool.transitivity eq eq₁) = Graph.transitivity (dioid-recur eq) (dioid-recur eq₁)
 labelled-graph-laws zero-commutativity = Graph.+commutativity
 labelled-graph-laws (left-identity {r = r}) with r
 ... | Bool.true = Graph.*left-identity