diff --git a/services/galley/src/Galley/API/MLS/SubConversation.hs b/services/galley/src/Galley/API/MLS/SubConversation.hs
index d22bad99d5e..a3c4538a4cb 100644
--- a/services/galley/src/Galley/API/MLS/SubConversation.hs
+++ b/services/galley/src/Galley/API/MLS/SubConversation.hs
@@ -288,7 +288,12 @@ deleteLocalSubConversation qusr lcnvId scnvId dsc = do
     let (gid, epoch) = (cnvmlsGroupId &&& cnvmlsEpoch) (scMLSData sconv)
     unless (dscGroupId dsc == gid) $ throwS @'ConvNotFound
     unless (dscEpoch dsc == epoch) $ throwS @'MLSStaleMessage
+    -- NOTE(md): check if this creates external proposals, but my hunch is it
+    -- doesn't as it shouldn't (there's no one to commit them once the subconv
+    -- is deleted).
     Eff.removeAllMLSClients gid
+    -- TODO(md): Make sure to call the 'deleteAllProposals gid' action to remove
+    -- all uncommitted proposals as it makes no sense to keep them around.
 
     newGid <- Eff.makeFreshGroupId
 
diff --git a/services/galley/src/Galley/Cassandra/SubConversation.hs b/services/galley/src/Galley/Cassandra/SubConversation.hs
index ad143121146..5847c1cf9fc 100644
--- a/services/galley/src/Galley/Cassandra/SubConversation.hs
+++ b/services/galley/src/Galley/Cassandra/SubConversation.hs
@@ -30,11 +30,15 @@ import Galley.API.MLS.Types (SubConversation (..))
 import Galley.Cassandra.Conversation.MLS (lookupMLSClients)
 import qualified Galley.Cassandra.Queries as Cql
 import Galley.Cassandra.Store (embedClient)
+import Galley.Effects.ProposalStore
 import Galley.Effects.SubConversationStore (SubConversationStore (..))
 import Imports
 import Polysemy
 import Polysemy.Input
+import qualified UnliftIO.Exception as UnliftIO
 import Wire.API.Conversation.Protocol
+import Wire.API.Error
+import Wire.API.Error.Galley
 import Wire.API.MLS.CipherSuite
 import Wire.API.MLS.Group
 import Wire.API.MLS.PublicGroupState
@@ -103,7 +107,11 @@ listSubConversations cid = do
       )
 
 interpretSubConversationStoreToCassandra ::
-  Members '[Embed IO, Input ClientState] r =>
+  ( Member (Embed IO) r,
+    Member (ErrorS 'ConvNotFound) r,
+    Member (Input ClientState) r,
+    Member ProposalStore r
+  ) =>
   Sem (SubConversationStore ': r) a ->
   Sem r a
 interpretSubConversationStoreToCassandra = interpret $ \case
@@ -115,7 +123,17 @@ interpretSubConversationStoreToCassandra = interpret $ \case
   SetSubConversationEpoch cid sconv epoch -> embedClient $ setEpochForSubConversation cid sconv epoch
   DeleteGroupIdForSubConversation groupId -> embedClient $ deleteGroupId groupId
   ListSubConversations cid -> embedClient $ listSubConversations cid
-  DeleteSubConversation convId subConvId -> embedClient $ deleteSubConversation convId subConvId
+  DeleteSubConversation convId subConvId -> do
+    msub <- embedClient (selectSubConversation convId subConvId)
+    case msub of
+      Nothing -> do
+        let e = toWai @GalleyError 'ConvNotFound
+        -- TODO(md): finish throwing the error
+        mapToRuntimeError $ throwS @'ConvNotFound
+        embed @IO $ UnliftIO.throwIO e
+      Just sub -> do
+        deleteAllProposals . cnvmlsGroupId . scMLSData $ sub
+        embedClient $ deleteSubConversation convId subConvId
 
 --------------------------------------------------------------------------------
 -- Utilities