Alkylsilane-modified styrene-butadiene-styrene (SBS) polymers were prepared via two steps. In the first step, dimethylchlorosilane HSi(CH3)(2)Cl and SBS (Korea Kumho Petrochem. Co.; 35% vinyl content; M-w = 48,000, M-w/M-n = 1.05) were reacted in toluene using H2PtCl6 catalyst to produce a hydrosilylated polymer. In the second step, chloride in the silyl group was replaced with alkyl by reaction with RLi (R = Me, n-Bu). Under severe conditions, silylation occurs on all the double bonds of the polymer so that their elastomeric nature disappears. However, under mild conditions, selective hydrosilylation of the pendant vinyl groups of SBS could be accomplished so that the polymer retained its elastomeric properties with two low T-g values. These modified SBS polymers were easily dissolved in THF, and thus could be formed into film-type membranes by solvent casting on teflon plates. From the measurements of their gas permeability for O-2 and N-2, they showed higher selectivity for O-2 compared with the unmodified SBS. The permeability for oxygen was dependent on the size of the alkylsilyl group. Methylsilane and butylsilane-modified SBS membranes showed the higher permeability values of 31.2 and 37.5 barrer (1 barrer = 1 x 10(-10) cm(3) (STP) cm cm(-2) s(-1) cmHg(-1)) at 30 degrees C, respectively, compared with 2.39 barrer of the unmodified SBS. Furthermore, their selectivities for O-2 over N-2 were increased to 3.0 and 3.2, respectively, from 2.5 of the unmodified SBS case.