Silica-zirconia membranes having pore sizes in the range 0.5-2 nm, determined using water as a condensable gas (vapor) by nanopermporometry, were prepared by the sol-gel process, and used in gas permeation experiments. The permeability ratio of He/N-2 approached the Knudsen value (= 2.6) for pore sizes larger than 2 nm. It then decreased with decreasing pore size, probably because of an enhanced contribution of surface diffusion in the small pore, and showed a minimum at an approximate pore size of 1 rim. It then increased to approximately 50 for a membrane having pore sizes of 0.3 nm. For the case of He/SF6, the curve appears to shift to a larger Kelvin diameter, probably because of the larger molecular size of SF6 as well as adsorption. The effects of non-condensable gases (He and N-2) were examined using silica-zirconia membranes of 2 and 0.8 nm in pore size. The pore size distribution (PSD) curves measured by He and N-2 were in good agreement with each other for membranes having a pore size as large as 2 nm. On the other hand, for the case of porous membranes having small pore sizes, PSD curves measured using He were shifted to a smaller pore size, compared with those measured by N-2. This suggests the existence of micropores, which allowed the permeation of only He. Moreover, nanopermporometry was applied to MFI zeolite membranes to characterize selective (intracrystalline) and non-selective pores (intercrystalline) using hexane, and the data were in reasonable agreement with the observed separation performances. (C) 2003 Elsevier Science B.V. All rights reserved.