diff --git a/lib/extensions.py b/lib/extensions.py
index bedb2463..bb8e3dd6 100644
--- a/lib/extensions.py
+++ b/lib/extensions.py
@@ -69,13 +69,11 @@ except ImportError:
 
 """Isolation level values."""
 ISOLATION_LEVEL_AUTOCOMMIT          = 0
+ISOLATION_LEVEL_READ_UNCOMMITTED    = 4
 ISOLATION_LEVEL_READ_COMMITTED      = 1
 ISOLATION_LEVEL_REPEATABLE_READ     = 2
 ISOLATION_LEVEL_SERIALIZABLE        = 3
 
-# PostgreSQL internally converts uncommited to commited
-ISOLATION_LEVEL_READ_UNCOMMITTED = ISOLATION_LEVEL_READ_COMMITTED
-
 """psycopg connection status values."""
 STATUS_SETUP    = 0
 STATUS_READY    = 1
diff --git a/psycopg/connection.h b/psycopg/connection.h
index 7f512a18..9a9ea420 100644
--- a/psycopg/connection.h
+++ b/psycopg/connection.h
@@ -32,6 +32,13 @@
 extern "C" {
 #endif
 
+/* isolation levels */
+#define ISOLATION_LEVEL_AUTOCOMMIT          0
+#define ISOLATION_LEVEL_READ_UNCOMMITTED    4
+#define ISOLATION_LEVEL_READ_COMMITTED      1
+#define ISOLATION_LEVEL_REPEATABLE_READ     2
+#define ISOLATION_LEVEL_SERIALIZABLE        3
+
 /* connection status */
 #define CONN_STATUS_SETUP       0
 #define CONN_STATUS_READY       1
diff --git a/psycopg/connection_int.c b/psycopg/connection_int.c
index 41b73f19..70465132 100644
--- a/psycopg/connection_int.c
+++ b/psycopg/connection_int.c
@@ -35,15 +35,16 @@
 #include <string.h>
 
 /* Mapping from isolation level name to value exposed by Python.
- * Only used for backward compatibility by the isolation_level property */
-
+ *
+ * Note: ordering matters: to get a valid pre-PG 8 level from one not valid,
+ * we increase a pointer in this list by one position. */
 const IsolationLevel conn_isolevels[] = {
-    {"",                    0}, /* autocommit */
-    {"read committed",      1},
-    {"read uncommitted",    1}, /* comes after to report real level */
-    {"repeatable read",     2},
-    {"serializable",        3},
-    {"default",            -1},
+    {"",                    ISOLATION_LEVEL_AUTOCOMMIT},
+    {"read uncommitted",    ISOLATION_LEVEL_READ_UNCOMMITTED},
+    {"read committed",      ISOLATION_LEVEL_READ_COMMITTED},
+    {"repeatable read",     ISOLATION_LEVEL_REPEATABLE_READ},
+    {"serializable",        ISOLATION_LEVEL_SERIALIZABLE},
+    {"default",            -1},     /* never to be found on the server */
     { NULL }
 };
 
@@ -1041,8 +1042,10 @@ conn_switch_isolation_level(connectionObject *self, int level)
 
     /* use only supported levels on older PG versions */
     if (self->server_version < 80000) {
-        if (level == 2)
-            level = 3;
+        if (level == ISOLATION_LEVEL_READ_UNCOMMITTED)
+            level = ISOLATION_LEVEL_READ_COMMITTED;
+        else if (level == ISOLATION_LEVEL_REPEATABLE_READ)
+            level = ISOLATION_LEVEL_SERIALIZABLE;
     }
 
     if (-1 == (curr_level = conn_get_isolation_level(self))) {
diff --git a/psycopg/connection_type.c b/psycopg/connection_type.c
index 02bc4da5..693de3ce 100644
--- a/psycopg/connection_type.c
+++ b/psycopg/connection_type.c
@@ -405,9 +405,9 @@ _psyco_conn_parse_isolevel(connectionObject *self, PyObject *pyval)
     if (PyInt_Check(pyval)) {
         long level = PyInt_AsLong(pyval);
         if (level == -1 && PyErr_Occurred()) { goto exit; }
-        if (level < 1 || level > 3) {
+        if (level < 1 || level > 4) {
             PyErr_SetString(PyExc_ValueError,
-                "isolation_level must be between 1 and 3");
+                "isolation_level must be between 1 and 4");
             goto exit;
         }
 
@@ -437,7 +437,8 @@ _psyco_conn_parse_isolevel(connectionObject *self, PyObject *pyval)
 
     /* use only supported levels on older PG versions */
     if (isolevel && self->server_version < 80000) {
-        if (isolevel->value == 1 || isolevel->value == 3) {
+        if (isolevel->value == ISOLATION_LEVEL_READ_UNCOMMITTED
+            || isolevel->value == ISOLATION_LEVEL_REPEATABLE_READ) {
             ++isolevel;
         }
     }
diff --git a/tests/test_connection.py b/tests/test_connection.py
index 0a791449..a887313a 100755
--- a/tests/test_connection.py
+++ b/tests/test_connection.py
@@ -206,6 +206,7 @@ class IsolationLevelsTestCase(unittest.TestCase):
 
         levels = [
             (None, psycopg2.extensions.ISOLATION_LEVEL_AUTOCOMMIT),
+            ('read uncommitted', psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED),
             ('read committed', psycopg2.extensions.ISOLATION_LEVEL_READ_COMMITTED),
             ('repeatable read', psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ),
             ('serializable', psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE),
@@ -215,8 +216,10 @@ class IsolationLevelsTestCase(unittest.TestCase):
 
             # the only values available on prehistoric PG versions
             if conn.server_version < 80000:
-                if level == psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ:
-                    name, level = ('serializable', psycopg2.extensions.ISOLATION_LEVEL_SERIALIZABLE)
+                if level in (
+                        psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED,
+                        psycopg2.extensions.ISOLATION_LEVEL_REPEATABLE_READ):
+                    name, level = levels[levels.index((name, level)) + 1]
 
             self.assertEqual(conn.isolation_level, level)
 
@@ -768,11 +771,13 @@ class TransactionControlTests(unittest.TestCase):
         self.assertEqual(cur.fetchone()[0], 'read committed')
         self.conn.rollback()
 
-        # 'read uncommitted' is internally translated to 'read committed'
         self.conn.set_session(
             isolation_level=psycopg2.extensions.ISOLATION_LEVEL_READ_UNCOMMITTED)
         cur.execute("SHOW default_transaction_isolation;")
-        self.assertEqual(cur.fetchone()[0], 'read committed')
+        if self.conn.server_version > 80000:
+            self.assertEqual(cur.fetchone()[0], 'read uncommitted')
+        else:
+            self.assertEqual(cur.fetchone()[0], 'read committed')
         self.conn.rollback()
 
     def test_set_isolation_level_str(self):