The hydrothermal stability of microporous niobia-silica membranes was investigated and compared with silica membranes. The membranes were exposed to hydrothermal conditions at 150 and 200 degrees C for 70 h. The change of pore structure before and after exposure to steam was probed by single-gas permeation measurements and nanopermporometry. The hydrothermal stability of the niobia-silica membrane was found to be higher than that of silica. After hydrothermal treatment at 200 degrees C, the hydrogen permeance of the niobia-silica top layer had declined by 32%, while the H-2 permeance of the silica top layer was reduced by 73%. The apparent activation energies of the H-2 permeance were 12.2 +/- 0.2 and 15.3 +/- 0.7 kJ mol(-1) for silica and niobia-silica, respectively. Nanopermporometry experiments on the silica membrane were in semi-quantitative agreement with the gas permeation data. The data suggest that densification of the top layer occurred predominantly in those areas with the highest convex curvatures, thereby increasing the effective transport path of helium and hydrogen across the membrane. (c) 2008 Elsevier B.V. All rights reserved.