diff --git a/psycopg/connection.h b/psycopg/connection.h
index a5553587..5b7c5d26 100644
--- a/psycopg/connection.h
+++ b/psycopg/connection.h
@@ -143,6 +143,11 @@ HIDDEN int  conn_tpc_begin(connectionObject *self, XidObject *xid);
     "in asynchronous mode");                                   \
     return NULL; }
 
+#define EXC_IF_TPC_BEGIN(self, cmd) if ((self)->tpc_xid) {  \
+    PyErr_Format(ProgrammingError, "%s cannot be used "     \
+    "during a two-phase transaction", #cmd);                \
+    return NULL; }
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/psycopg/connection_type.c b/psycopg/connection_type.c
index d0e0c500..9cc32a2d 100644
--- a/psycopg/connection_type.c
+++ b/psycopg/connection_type.c
@@ -137,6 +137,7 @@ psyco_conn_commit(connectionObject *self, PyObject *args)
 {
     EXC_IF_CONN_CLOSED(self);
     EXC_IF_CONN_ASYNC(self, commit);
+    EXC_IF_TPC_BEGIN(self, commit);
 
     if (!PyArg_ParseTuple(args, "")) return NULL;
 
@@ -158,6 +159,7 @@ psyco_conn_rollback(connectionObject *self, PyObject *args)
 {
     EXC_IF_CONN_CLOSED(self);
     EXC_IF_CONN_ASYNC(self, rollback);
+    EXC_IF_TPC_BEGIN(self, rollback);
 
     if (!PyArg_ParseTuple(args, "")) return NULL;