The effect of a supercritical carbon dioxide (scCO(2)) treatment on the free volume of poly[1-(trimethylsilyl)-1-propyne] (PTMSP) and PTMSP-silica nanohybrid membranes was investigated with positron annihilation lifetime spectroscopy (PALS) and correlated with their performance in pervaporation. Treatment of dense unfilled PTMSP and silica filled PTMSP (2.5, 10 and 20 wt.% silica) membranes with scCO(2) at different pressures and temperatures, resulted in a clear increase in the mean free volume size and an enhanced performance in the pervaporative separation of an aqueous ethanol mixture. Specific permeation rate increases up to 76% were observed for the unfilled membranes treated with scCO(2) at 70 degrees C and 24 MPa. The scCO(2)-treatment could thus be seen as an effective alternative to PTMSP-silica (50 wt.%) membranes, which cause a 72% increase in specific permeation rate. Moreover, the ethanol concentration in the permeate increases significantly upon treating the membranes with scCO(2). (C) 2011 Elsevier B.V. All rights reserved.