Poly(I-trimethylsilyl-l-propyne) (PTMSP) membranes have been used to separate ethanol-water mixtures by pervaporation. This polyacetylene is known to present high affinity toward ethanol, showing high selectivity and ethanol permeation flux. The performance of this polymer in the separation of alcohol-water solutions has been evaluated over long periods (572 h) at a high temperature (75 degrees C) to examine the deterioration of the transport properties in the separation of 50 wt % ethanol-water solutions. Although PTMSP membranes present good characteristics for the separation of gases and liquid mixtures, their organic selectivity decrease with the operating time because of the relaxation processes of the polymeric chains, which affect the free volume of the polymer, the deterioration being more evident for concentrated solutions. The effects of the operation temperature on the characteristic parameters of pervaporation have also been studied to establish how this variable affects the performance of PTMSP membranes. The selectivity increases slightly with the operation temperature, but the effect of the temperature on the separation factor decreases as membranes are degraded with the operation time. (c) 2006 Wiley Periodicals, Inc.