Thin (0.5-5-mu m) palladium (Pd) membranes were prepared inside pores or on the surface of mesoporous (pore size 4-6 nm), sol-gel-derived gamma-Al2O3 supports by the chemical vapor deposition (CVD) method using palladium acetylacetonate and palladium chloride (together with hydrogen) as Pd precursors. When Pd is deposited in the form of metal plugs inside pores of gamma-Al2O3, He permeance of the composite membranes increases exponentially with temperature, indicating activated diffusion through the microporous metal deposit defects, while only modest permselectivity for H-2 is observed When Pd is deposited in the form of a metal film on the surface of gamma-Al2O3, He permeance is governed by Knudsen-viscous flow through meso/macroporous metal film pears to increase with palladium crystal grain size. The highest H-2 permeance and H-2:He permselectivity for membranes prepared in this study are about 1.0-2.0 X 10(-7) mol.m(-2).s(-1).Pa-1 and 200-300 at 300 degrees C, respectively.