A series of Si-modified membranes were prepared by chemical vapor deposition using 40 Angstrom gamma-alumina tubular membranes as supports. Their hydrogen permeance ranged from 0.028 to 17.6 m(3)/m(2) h atm and H-2/N-2 selectivity (permeance ratio) ranged from 12.6 to 72 at 600 degrees C. The selectivity to isobutane ranged from 40 to 240 at 300 degrees C and could be higher at a higher temperature. Compared with similar membranes documented in the literature, these membranes exhibited "order-of-magnitude" improvement in the permeance while maintaining a moderate selectivity. These membranes could be ideal for industrial gas separations and catalytic reactions handling a large volume of streams. Hindrance diffusion through micropores (i.e., similar to 5 Angstrom) and Knudsen diffusion through larger pores were suggested separation mechanisms for these modified membranes. These mechanisms coupled with hypothetical pore size distributions were tested satisfactorily with a wide range of permeation behaviors delivered by a series of membranes with different microporous structures. Specifically, they explained the subtle permeation difference between hydrogen and helium, and the relative contribution between hindrance and Knudsen diffusion for nitrogen. Separations of gas mixtures containing hydrogen were confirmed similar to the ideal separations determined by single components. The modified membranes were thermally stable at 600 degrees C. The hydrothermal stability test indicated that the membrane structure approached a new steady state immediately after exposing to moisture.