Brominated poly(1‐trimethylsilyl‐1‐propyne) (Br–PMSP), which was prepared by immersing a poly(1‐trimethylsilyl‐1‐propyne) membrane in bromine water, had a new endothermic peak between 70°C and 80°C in its DSC curves. After storage in air at room temperature,the endothermic peak shifted to the low temperature side with time, and occurred between 30°C and 40°C. A distinct change in slopes near the endothermic temperature determined by the DSC analysis was observed in the Arrhenius plots of the permeability coefficients for various gases in all Br–PMSP membranes. However, the slope's change did not clearly appear with storage under the same conditions. The Br–PMSP membrane had a pressure‐dependence of the permeability coefficients not only below the endothermic temperature, but also above one. The sorption behavior for propane in the Br–PMSP membrane was similar to that for other glassy polymers, and there was little change in the sorption behavior with time. In the case of carbon dioxide, a different behavior was observed at first; that is, the same isotherm shape in glassy polymers appeared below 20 atm, but it showed an upward curve above that. After storage in air, the upward behavior disappeared, and the isotherm showed the general shape in glassy polymers. © 1994 John Wiley & Sons, Inc.