The successful application of zeolite A membranes in the industrial market has thus far been restricted to the pervaporative dehydration of solvent streams in the pharmaceutical and engineering industries. Their application in gas separation processes remains elusive, largely due to the existence of uncontrollable, intercrystalline diffusion pathways in the boundary regions of neighbouring crystals. In this study the gas permeation characteristics of an unconventional, semicrystalline (70%) zeolite NaA layer, where the boundary phase is filled with amorphous aluminosilicate, was examined and compared to a traditional polycrystalline membrane. To this end, we used single gas permeation of H-2, N-2 and SF6 at two temperatures. In general, the permeances of all gases through the conventional layer increased in response to temperature, indicating a governing contribution of activated (gaseous) diffusion over the temperature interval tested. Typical permeances of H-2 were in the order of 4.8 x 10(-7) (23 degrees C) to 6.5 x 10(-7) mol m(-2) s(-1) Pa-1 (107 degrees C). For the semicrystalline layer, the same trends were observed for N-2 and SF6, while the permeance of H-2 decreased from 8.7 x 10(-7) (23 degrees C) to about 3.9 x 10(-7) mol m(-2) s(-1) Pa-1 at 107 degrees C. This was attributed to the expansion of the crystal/amorphous interface at higher temperature. By comparing the permselectivity values with the respective Knudsen factors, it was shown that diffusion through the semicrystalline layer, at lower temperature, was predominantly based on molecular sieving (PS H-2/SF6 = 63.8), which yielded higher selectivities than the traditional membrane under the same conditions (PS H-2/SF6 = 11.4). However, at higher temperature the H-2/SF6 permselectivity of the crystalline layer (5.7) was somewhat higher than that of the sernicrystalline membrane (5.2). Based on theoretical considerations, it was concluded that the crystal/amorphous interface in the semicrystalline membrane constituted a denser closure of the boundary interface, which could be attributed to a lower charge barrier presented by the amorphous phase (Si/Al > 1), but this integrity was lost at higher temperature due to the thermal instability of the amorphous material. The results therefore suggested that future research should focus on lowering the intercrystalline charge barrier during synthesis, which could reduce the intercrystalline porosity and improve the gas separation properties of NaA membranes in general. (c) 2006 Elsevier B.V. All rights reserved.