A-type zeolite membranes were synthesized on a porous support tube by a hydrothermal process. The membranes were polycrystalline, and the thickness was in the range of 0.4-3.8 mu m. They were stable against repeated temperature changes over the range 35-300 degrees C. Permeances of the membranes, determined using He, H2O, H-2, CO2, O-2, N-2, CH4 and C3H8, were dependent on the size of permeate, and the maximum H-2/N-2 separation factor was 4.8 for a mixed H-2-N-2 feed. This suggests the permeation was, at least in part, affected by a molecular sieving mechanism, although the membranes contained some pores larger than the structural pores of the zeolite.