diff --git a/cashu/mint/crud.py b/cashu/mint/crud.py
index 313dace..aa53bd5 100644
--- a/cashu/mint/crud.py
+++ b/cashu/mint/crud.py
@@ -29,6 +29,18 @@ class LedgerCrud:
 
         return await invalidate_proof(*args, **kwags)
 
+    async def get_proofs_pending(*args, **kwags):
+
+        return await get_proofs_pending(*args, **kwags)
+
+    async def set_proof_pending(*args, **kwags):
+
+        return await set_proof_pending(*args, **kwags)
+
+    async def unset_proof_pending(*args, **kwags):
+
+        return await unset_proof_pending(*args, **kwags)
+
     async def store_keyset(*args, **kwags):
 
         return await store_keyset(*args, **kwags)
@@ -102,6 +114,55 @@ async def invalidate_proof(
     )
 
 
+async def get_proofs_pending(
+    db: Database,
+    conn: Optional[Connection] = None,
+):
+
+    rows = await (conn or db).fetchall(
+        f"""
+        SELECT * from {table_with_schema(db, 'proofs_pending')}
+        """
+    )
+    return [Proof(**r) for r in rows]
+
+
+async def set_proof_pending(
+    db: Database,
+    proof: Proof,
+    conn: Optional[Connection] = None,
+):
+
+    # we add the proof and secret to the used list
+    await (conn or db).execute(
+        f"""
+        INSERT INTO {table_with_schema(db, 'proofs_pending')}
+          (amount, C, secret)
+        VALUES (?, ?, ?)
+        """,
+        (
+            proof.amount,
+            str(proof.C),
+            str(proof.secret),
+        ),
+    )
+
+
+async def unset_proof_pending(
+    proof: Proof,
+    db: Database,
+    conn: Optional[Connection] = None,
+):
+
+    await (conn or db).execute(
+        f"""
+        DELETE FROM {table_with_schema(db, 'proofs_pending')}
+        WHERE secret = ?
+        """,
+        (str(proof["secret"]),),
+    )
+
+
 async def store_lightning_invoice(
     db: Database,
     invoice: Invoice,
diff --git a/cashu/mint/ledger.py b/cashu/mint/ledger.py
index 8f328ce..316a831 100644
--- a/cashu/mint/ledger.py
+++ b/cashu/mint/ledger.py
@@ -258,7 +258,10 @@ class Ledger:
         return ok, preimage
 
     async def _invalidate_proofs(self, proofs: List[Proof]):
-        """Adds secrets of proofs to the list of knwon secrets and stores them in the db."""
+        """
+        Adds secrets of proofs to the list of known secrets and stores them in the db.
+        Removes proofs from pending table.
+        """
         # Mark proofs as used and prepare new promises
         proof_msgs = set([p.secret for p in proofs])
         self.proofs_used |= proof_msgs
@@ -266,6 +269,38 @@ class Ledger:
         for p in proofs:
             await self.crud.invalidate_proof(proof=p, db=self.db)
 
+        # delete proofs from pending list
+        await self._unset_proofs_pending(proofs)
+
+    async def _set_proofs_pending(self, proofs: List[Proof]):
+        """
+        If none of the proofs is in the pending table (_validate_proofs_pending), adds proofs to
+        the list of pending proofs or removes them. Used as a mutex for proofs.
+        """
+        # first we check whether these proofs are pending aready
+        await self._validate_proofs_pending(proofs)
+        for p in proofs:
+            await self.crud.set_proof_pending(proof=p, db=self.db)
+
+    async def _unset_proofs_pending(self, proofs: List[Proof]):
+        """Deletes proofs from pending table."""
+        # we try: except: this block in order to avoid that any errors here
+        # could block the _invalidate_proofs() call that happens afterwards.
+        try:
+            for p in proofs:
+                await self.crud.unset_proof_pending(proof=p, db=self.db)
+        except Exception as e:
+            print(e)
+            pass
+
+    async def _validate_proofs_pending(self, proofs: List[Proof]):
+        """Checks if any of the provided proofs is in the pending proofs table. Raises exception for at least one match."""
+        proofs_pending = await self.crud.get_proofs_pending(db=self.db)
+        for p in proofs:
+            for pp in proofs_pending:
+                if p.secret == pp.secret:
+                    raise Exception("proofs are pending.")
+
     # Public methods
     def get_keyset(self, keyset_id: str = None):
         keyset = self.keysets.keysets[keyset_id] if keyset_id else self.keyset
@@ -308,6 +343,10 @@ class Ledger:
 
     async def melt(self, proofs: List[Proof], invoice: str):
         """Invalidates proofs and pays a Lightning invoice."""
+
+        # validate and set proofs as pending
+        await self._set_proofs_pending(proofs)
+
         # Verify proofs
         if not all([self._verify_proof(p) for p in proofs]):
             raise Exception("could not verify proofs.")
@@ -354,6 +393,10 @@ class Ledger:
         keyset: MintKeyset = None,
     ):
         """Consumes proofs and prepares new promises based on the amount split."""
+
+        # set proofs as pending
+        await self._set_proofs_pending(proofs)
+
         total = sum_proofs(proofs)
 
         # verify that amount is kosher
diff --git a/cashu/mint/migrations.py b/cashu/mint/migrations.py
index 51a6459..4b6b5e3 100644
--- a/cashu/mint/migrations.py
+++ b/cashu/mint/migrations.py
@@ -128,3 +128,21 @@ async def m004_keysets_add_version(db: Database):
     await db.execute(
         f"ALTER TABLE {table_with_schema(db, 'keysets')} ADD COLUMN version TEXT"
     )
+
+
+async def m005_pending_proofs_table(db: Database) -> None:
+    """
+    Store pending proofs.
+    """
+    await db.execute(
+        f"""
+            CREATE TABLE IF NOT EXISTS {table_with_schema(db, 'proofs_pending')} (
+                amount INTEGER NOT NULL,
+                C TEXT NOT NULL,
+                secret TEXT NOT NULL,
+
+                UNIQUE (secret)
+
+            );
+        """
+    )