The present paper describes the synthesis and properties of porous glass membranes surface-modified using organosilane compounds with different hydrocarbon lengths (CnH2n+1(CH3)(2)SiCl; n = 1, 3, 8, 18). Single gas permeation through the membranes was measured at 298, 333 and 373 K using He, N-2, CO2, CH4, C2H6, C3H8, n-C4H10 and i-C4H10. The membranes showed highly selective separation of hydrocarbon gases and high permeances. Hydrocarbon gas selectivity increased with increasing carbon chain length in the hydrocarbon gases and in the organosilane compounds used. For C18 membrane (surface-modified with dimethyloctadecylchlorosilane), the ratio of the permeances, n-C4H10/N-2 was 66.6 at 298 K, 22.5 at 333 K and 14.0 at 373 K. The value at 298 K is around 100 times the theoretical Knudsen value (n-C4H10/N-2 = 0.69). Binary gas permeation at 373 K was also investigated using a gas mixture of 90% CH4-10% n-C4H10. The ideal separation factor of n-C4H10/CH4 through C18 membrane was 8.8, higher than in single gas measurement due to hindering by adsorbed molecules. To evaluate the adsorption affinities of the hydrocarbon gases CH4, C2H6, C3H8, n-C4H10 and i-C4H10 on surface-modified membranes at high temperature, dynamic flow adsorption measurement was performed at 373 K. Due to the low relative pressures, all the isotherms obtained followed Henry's adsorption equation almost exactly. Amounts of hydrocarbon gases adsorbed increased with increasing carbon chain length in the organosilane compounds. These results confirmed that surface modification of porous glass membranes using a covalently bonded molecular monolayer is a special way of altering membrane performance to introduce various organic functionalities. The main frame of surface-modified membrane was inorganic glass with rigid pore. which were modified by the organic monolayer. The permeation paths of gas molecules were maintained due to the small influence of the micro-Brownian motion of the organic chains. These surface-modified membranes, therefore, displayed high thermal stability and high permeance as well as organic functionality (high selectivity).