A thermostatic valve comprising a valve head and a flat, annular, elastomeric sealing element at the valve head, a valve spring mechanically biasing the valve member against a valve seat and a temperature-sensitive expansible element the valve member displacing the valve member against the valve spring, characterized by the valve head comprising at its circumference an annular groove, the flat annular sealing element being stamped out of an elastomer which swells in the presence of water, in particular an EPDM material, and exhibiting a thickness which is equal to or slightly less than the width of the annular groove.