Permselective membranes were prepared by grafting of dimethylsiloxane to poly(styrene-co-maleic anhydride) membrane with the reaction of an amino group in DMS and a maleic anhydride group in the copolymer. The grafting reaction penetrated into the copolymer membrane with sharp boundary fronts formed. The gas permeation characteristics of the membrane obtained were close to the values of poly( dimethylsiloxane) membrane, The permeability coefficient was on the order of magnitude of 10(-8) [cm(3)(STP) cm/cm(2) s cmHg] for O-2 gas, and gave an ideal separation factor of 2.3 for the O-2-N-2 system. The gas permeability was controlled with change of the dimethylsiloxane content and the molecular weight of dimethylsiloxane in the graft membrane. High permselective membranes were prepared with ease by an application of the above grafting technique onto commercial porous membranes, The permeation coefficients of O-2 through the membranes were 1.5-6.8 x 10(-8) and the ideal separation factors were improved to 4.5-2.4 for the O2-N-2 system.