An ultrathin, high-flux composite membrane for use in gas separation. The membrane comprises a metal layer, typically 500.ANG. or less thick on a microporous support. The support may be as asymmetric Loeb-Sourirajan type membrane or a two-layer composite of a microporous substrate with a very thin sealing layer. The support gives mechanical strength to the membrane but does not contribute significantly to the selectivity. Optionally the metal layer may be overcoated with a second polymeric sealing layer to plug any defects and to protect the fragile surface from mechanical damage. The membrane is particularly useful in the purfication of hydrogen.