A B S T R A C T Pd–Ag based membranes supported on porous a-Al 2 O 3 (doped with yttria-stabilized zirconia) were studied for hydrogen selective separation. Magnetron sputtering technique was employed for the synthesis of thin film membranes. The hydrogen permeation flux is affected by the membrane columnar structure, which is formed during deposition. From scanning electron microscopy analysis, it was observed that different sputtering deposition pressures lead to distinct columnar structure growth. X-ray diffraction patterns provided evidence of a Pd–Ag solid solution with an average crystallite domain size of 21 nm, whose preferential growth can be altered by the deposition pressure. The gas-permeation results have shown that the Pd–Ag membrane supported on porous a-Al 2 O 3 is selective toward H 2. For optimized membrane synthesis conditions, the permeance toward N 2 is 0.076 Â 10 À6 mol m À2 s À1 Pa À1 at room temperature, whereas for a pressure difference of 300 kPa the H 2-flux is of the order of ca. 0.21 mol m À2 s À1 , which corresponds to a permeance of 0.71 Â10 À6 mol m À2 s À1 Pa À1 , yielding a selectivity of a (H 2 /N 2) = 10. These findings suggest that the membrane has a reasonable capacity to selectively permeate this gas. ã