The reverse selectivity of inorganic membranes can be enhanced by surface modification using long hydrocarbon chain silanes. Improvements in n-C4H10selectivity of Vycor porous glass over N2and CH4were obtained by sequentially modifying it with a C18monochlorosilane (OCS) followed by a C18trichlorosilane (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/N2and n-C4H10/CH4ideal 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-C4H10over N2and CH4after 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 nm pore diameter -alumina ultrafilter substrate. The mixed-gas n-butane permeance for this membrane was 2 × 10-4cm3(STP) cm-2cm Hg-1s-1(6.7 × 10-8mol m-2s-1Pa-1). [ABSTRACT FROM AUTHOR]