Rhodium and iridium were used as novel membrane materials for hydrogen separation. The composite membranes consisting of these metals dispersed in the pores of an asymmetric porous alumina were prepared by the CVD technique. Rh(acac)(3) and Ir(acac)3 complexes were selected as the CVD precursors. The hydrogen fluxes of the rhodium and iridium membranes were 0.155 and 0.178 mol m(-2) s(-1) at 773 K and the transmembrane pressure difference of 98 kPa, respectively, which were comparable to those of the palladium composite membranes prepared by electroless plating and CVD. The ideal permselectivities of hydrogen to nitrogen were 80 and 93 for rhodium and iridium, respectively. Hydrogen molecules adsorbed on metals dissociated into the atomic form and then the atoms diffused through the metal layer. The CVD membranes gave similar hydrogen permeation coefficients except for palladium. (C) 2000 Elsevier Science B.V. All rights reserved.