Polymeric membranes have not been practical for application to high temperature processes, due to the thermal instability of most polymers. New materials are being developed with higher glass transition temperatures and a greater degree of thermal stability. Use of these polymers to perform gas separations at higher temperatures is promising; however, the performance of the membrane as the process approaches the polymer’s glass transition temperature is unknown. This study was conducted to explore this issue. Gas flux and helium/nitrogen ideal selectivities through heat treated integrally-skinned asymmetric polysulfone membranes were measured. Membrane morphology was also evaluated through bubble point tests and SEM micrographs. Experiments demonstrated that as the polymer is exposed to temperatures approaching the polymer glass transition temperature, internal pores begin to collapse, causing both the gas flux and selectivity to decrease.