Hydrogen separation on microporous methyltriethoxysilane-templating silica composite/alpha-alumina membranes (below MTES membrane) was studied using three binary gas mixtures: H-2/N-2, H-2/CO2, and H-2/CH4. The characteristics of unsteady and steadystate permeation/separation on the MTES membrane were compared to each other. Although permeation flux in the H-2/N-2 mixture was comparatively low, H-2 selectivity was high (H-2/N-2 SF approximate to 30-60). On the contrary, the H-2/CO2 mixture showed high permeation flux but low H2 selectivity (H-2/CO2 SF approximate to 1.5-6.5). The H-2/CH4 mixture showed a large difference between permselectivity (28-48) and separation factor (1022). Results from this study revealed that it was difficult to predict the separation factor using the one-component permeation ratio (permselectivity) over the experimental range tested. These separation characteristics could be primarily ascribed to the molecular size and structure of each gas, which likely contributed to steric hindrance or molecular sieving within the membrane pore. In addition, the adsorption affinity of each molecule on the membrane surface acted as a key factor in separation performance because it significantly influenced surface diffusion. The generalized Maxwell-Stefan model incorporating the dust gas model, and the Langmuir model could successfully predict the transient and steady-state permeation/separation. (c) 2007 American Institute of Chemical Engineers AIChE J, 53: 3125-3136, 2007.