Nanocomposite membranes of polydimethylsiloxane (PDMS) with different amounts of fumed silica were synthesized over a porous support of polyacrylonitrile. The prepared membranes were characterized using atomic force microscopy and thermal gravimetric analysis. AFM results indicated the nanoscale dispersion of silica particles in polymer matrix. TGA results indicated that increasing the silica content enhances the thermal stability of membranes. Permeation of methane and n-butane single gases at different upstream pressures and also permeation of a gas mixture containing 3 mol% of n-butane in methane were studied and discussed. The results showed that the nanocomposite membrane containing 11 wt% of fumed silica exhibited 38% increment in permeability of n-butane and simultaneously 30% increase in selectivity of n-butane over methane. The unusual property enhancement that contradicts to that of conventional filled polymer systems has been discussed based on density of membranes and free volume concept. (c) 2012 Elsevier B.V. All rights reserved.