Perfluorosulfonic acid (PFSA) electrolyte heat-treated at elevated glass transition temperature (T(g), 140-290 degrees C) with alkaline metal ions as the sulfonic group protector have been used to prepare stable PFSA/polytetrafluoroethylene (ePTFE) polymer electrolyte membrane for fuel cells. Thermal mechanical analysis (TMA) and X-ray diffraction results revealed that the PFSA resin with alkaline metal ions (Li, Na, and K) exhibits a significant increase of T(g), as well as an improved crystalline structure after heat treatment at T(g). Resin solubility measurement indicated the heat-treated PFSA show a better solvent resistance which favors membrane stability. The effect of ionic modification on PFSA/ePTFE composite PEMs performances, such as gas crossover, shrinkage stress, proton conductivity, and mechanical properties has been investigated in details. In general, the heat-treated PFSA/ePTFE composite PEMs showed lower dissolved fraction in water-alcohol solvents and better dimensional stability. The increased heat-treated temperature also enhanced the membrane performance because of the improved interface compatibility between PFSA resin and FIFE. The PFSA/ePTFE membrane prepared from heat-treated PFSA in Na-form at 290 degrees C (T(g) of Na-form PFSA) increased the performance up to 0.744 V@600 rnA cm(-2), significantly higher than those of other PEMs because of the T(g) of Na-PFSA were close to the T(g) of PTFE. (C) 2011 Elsevier B.V. All rights reserved.