diff --git a/ouroboros-network/changelog.d/20251119_074504_isovector_fix_diffusion_mode.md b/ouroboros-network/changelog.d/20251119_074504_isovector_fix_diffusion_mode.md
new file mode 100644
index 0000000000..67e3dd277b
--- /dev/null
+++ b/ouroboros-network/changelog.d/20251119_074504_isovector_fix_diffusion_mode.md
@@ -0,0 +1,19 @@
+<!--
+A new scriv changelog fragment.
+
+Uncomment the section that is right (remove the HTML comment wrapper).
+For top level release notes, leave all the headers commented out.
+-->
+
+
+### Breaking
+
+- Changed ToJSON for DiffusionMode to agree with the topology file syntax. ReadJSON was already correct.
+
+
+<!--
+### Non-Breaking
+
+- A bullet item for the Non-Breaking category.
+
+-->
diff --git a/ouroboros-network/orphan-instances/Ouroboros/Network/OrphanInstances.hs b/ouroboros-network/orphan-instances/Ouroboros/Network/OrphanInstances.hs
index 275fad7d22..8eb49ea336 100644
--- a/ouroboros-network/orphan-instances/Ouroboros/Network/OrphanInstances.hs
+++ b/ouroboros-network/orphan-instances/Ouroboros/Network/OrphanInstances.hs
@@ -117,7 +117,8 @@ instance FromJSON DiffusionMode where
       _ -> fail "Parsing NodeDiffusionMode failed: can be either 'InitiatorOnly' or 'InitiatorAndResponder'"
 
 instance ToJSON DiffusionMode where
-  toJSON = String . pack . show
+  toJSON InitiatorOnlyDiffusionMode         = "InitiatorOnly"
+  toJSON InitiatorAndResponderDiffusionMode = "InitiatorAndResponder"
 
 -- | Does not use the 'FromJSON' instance of 'RootConfig', so that
 -- 'accessPoints', 'advertise', 'valency' and 'warmValency' fields are attached