We have investigated the effect of the recast temperature, i.e., heat-treatment of a polymer electrolyte, on the diffusion coefficient and solubility of oxygen in the electrolyte and also on the morphological properties of recast ion-exchange membranes for improving the cathode activity in polymer electrolyte fuel cells. Based on chronoamperometric measurements, the diffusion coefficient and solubility of oxygen were found to be deeply affected by the recast temperature. The diffusion coefficient increased with the decreasing recast temperature, while the solubility had the opposite tendency. The water uptakes and ionic cluster size also varied with the recast temperature. Based on the X-ray measurements, it is considered that the differences in the mass transport parameters, the cluster sizes, and water uptakes are due to the growth of the clusters and the crystallinity in the electrolyte. (C) 2004 The Electrochemical Society.