diff --git a/lib/sql.py b/lib/sql.py
index ae7c4bab..0bd1e31b 100644
--- a/lib/sql.py
+++ b/lib/sql.py
@@ -23,6 +23,8 @@
 # FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
 # License for more details.
 
+import re
+import collections
 from psycopg2 import extensions as ext
 
 
@@ -187,8 +189,102 @@ class Placeholder(Composible):
             return "%s"
 
 
-def compose(sql, args=()):
-    raise NotImplementedError
+re_compose = re.compile("""
+    %                           # percent sign
+    (?:
+        ([%s])                  # either % or s
+        | \( ([^\)]+) \) s      # or a (named)s placeholder (named captured)
+    )
+    """, re.VERBOSE)
+
+
+def compose(sql, args=None):
+    phs = list(re_compose.finditer(sql))
+
+    # check placeholders consistent
+    counts = {'%': 0, 's': 0, None: 0}
+    for ph in phs:
+        counts[ph.group(1)] += 1
+
+    npos = counts['s']
+    nnamed = counts[None]
+
+    if npos and nnamed:
+        raise ValueError(
+            "the sql string contains both named and positional placeholders")
+
+    elif npos:
+        if not isinstance(args, collections.Sequence):
+            raise TypeError(
+                "the sql string expects values in a sequence, got %s instead"
+                % type(args).__name__)
+
+        if len(args) != npos:
+            raise ValueError(
+                "the sql string expects %s values, got %s" % (npos, len(args)))
+
+        return _compose_seq(sql, phs, args)
+
+    elif nnamed:
+        if not isinstance(args, collections.Mapping):
+            raise TypeError(
+                "the sql string expects values in a mapping, got %s instead"
+                % type(args))
+
+        return _compose_map(sql, phs, args)
+
+    else:
+        if not isinstance(args, collections.Sequence) and args:
+            raise TypeError(
+                "the sql string expects no value, got %s instead" % len(args))
+        # If args are a mapping, no placeholder is an acceptable case
+
+        # Convert %% into %
+        return _compose_seq(sql, phs, ())
+
+
+def _compose_seq(sql, phs, args):
+    rv = []
+    j = 0
+    for i, ph in enumerate(phs):
+        if i:
+            rv.append(SQL(sql[phs[i - 1].end():ph.start()]))
+        else:
+            rv.append(SQL(sql[0:ph.start()]))
+
+        if ph.group(1) == 's':
+            rv.append(args[j])
+            j += 1
+        else:
+            rv.append(SQL('%'))
+
+    if phs:
+        rv.append(SQL(sql[phs[-1].end():]))
+    else:
+        rv.append(sql)
+
+    return Composed(rv)
+
+
+def _compose_map(sql, phs, args):
+    rv = []
+    for i, ph in enumerate(phs):
+        if i:
+            rv.append(SQL(sql[phs[i - 1].end():ph.start()]))
+        else:
+            rv.append(SQL(sql[0:ph.start()]))
+
+        if ph.group(2):
+            rv.append(args[ph.group(2)])
+        else:
+            rv.append(SQL('%'))
+
+    if phs:
+        rv.append(SQL(sql[phs[-1].end():]))
+    else:
+        rv.append(sql)
+
+    return Composed(rv)
 
 
 # Alias
diff --git a/psycopg/cursor_type.c b/psycopg/cursor_type.c
index a7303c68..485c69ec 100644
--- a/psycopg/cursor_type.c
+++ b/psycopg/cursor_type.c
@@ -267,10 +267,34 @@ _mogrify(PyObject *var, PyObject *fmt, cursorObject *curs, PyObject **new)
     return 0;
 }
 
+/* Return 1 if `obj` is a `psycopg2.sql.Composible` instance, else 0
+ * Set an exception and return -1 in case of error.
+ */
+RAISES_NEG static int
+_curs_is_composible(PyObject *obj)
+{
+    int rv = -1;
+    PyObject *m = NULL;
+    PyObject *comp = NULL;
+
+    if (!(m = PyImport_ImportModule("psycopg2.sql"))) { goto exit; }
+    if (!(comp = PyObject_GetAttrString(m, "Composible"))) { goto exit; }
+    rv = PyObject_IsInstance(obj, comp);
+
+exit:
+    Py_XDECREF(comp);
+    Py_XDECREF(m);
+    return rv;
+
+}
+
 static PyObject *_psyco_curs_validate_sql_basic(
     cursorObject *self, PyObject *sql
   )
 {
+    PyObject *rv = NULL;
+    int comp;
+
     /* Performs very basic validation on an incoming SQL string.
        Returns a new reference to a str instance on success; NULL on failure,
        after having set an exception. */
@@ -278,26 +302,32 @@ static PyObject *_psyco_curs_validate_sql_basic(
     if (!sql || !PyObject_IsTrue(sql)) {
         psyco_set_error(ProgrammingError, self,
                          "can't execute an empty query");
-        goto fail;
+        goto exit;
     }
 
     if (Bytes_Check(sql)) {
         /* Necessary for ref-count symmetry with the unicode case: */
         Py_INCREF(sql);
+        rv = sql;
     }
     else if (PyUnicode_Check(sql)) {
-        if (!(sql = conn_encode(self->conn, sql))) { goto fail; }
+        if (!(rv = conn_encode(self->conn, sql))) { goto exit; }
+    }
+    else if (0 != (comp = _curs_is_composible(sql))) {
+        if (comp < 0) { goto exit; }
+        if (!(rv = PyObject_CallMethod(sql, "as_string", "O", self->conn))) {
+            goto exit;
+        }
     }
     else {
         /* the  is not unicode or string, raise an error */
         PyErr_SetString(PyExc_TypeError,
-                        "argument 1 must be a string or unicode object");
-        goto fail;
+            "argument 1 must be a string or unicode object");
+        goto exit;
     }
 
-    return sql; /* new reference */
-    fail:
-        return NULL;
+exit:
+    return rv;
 }
 
 /* Merge together a query string and its arguments.