The reverse selectivity of inorganic membranes can be enhanced by surface modification using long hydrocarbon chain silanes. Improvements in n-C4H10, selectivity of Vycor porous glass over N-2 and CH4 were obtained by sequentially modifying it with a C-18 monochlorosilane (OCS) followed by a C-18 trichlorosilane (ODS). The membranes were tested for pure and mixed gases at different feed pressures. The dual-modified membranes have higher ideal, or pure-gas, selectivities than membranes modified using OCS alone. The n-C4H10/N-2 and n-C4H10/CH4 ideal selectivities for the dual-modified membrane were 88 and 35, respectively, as compared to values of 13 and 7, respectively, for the OCS-modified membrane. Adsorption experiments showed an increase in the relative adsorption of n-C4H10 over N-2 and CH4 after dual surface modification of Vycor glass. Nuclear magnetic resonance, X-ray photoelectron spectroscopy, and thermogravimetric analysis techniques were used to confirm the presence of silanes on the surface of the substrates. A high flux of n-butane was measured for a dual-modified membrane fabricated using a 5 run pore diameter y-alumina ultrafilter substrate. The mixed-gas n-butane permeance for this membrane was 2 x 10(-4) cm(3) (STP) CM-2 CM Hg-1 S-1 (6.7 x 10(-8) mol m(-2) s(-1) Pa-1).