Composite gas separation membranes with a 0.1-3 mu m thick film of Hyflon AD 60X on a porous hollow fibre ultrafiltration membrane support of amorphous poly(ether ether ketone), were prepared. The influence of the coating conditions on the film thickness and on the pure gas transport properties was studied. The gas transport properties were related to the film thickness and to the morphology of the supporting UF membrane.