Mass transport parameters are determined for oxygen reduction in two different proton exchange membranes: (1) Nafion(R) 117 (Du Pont)-equivalent weight 1100 g mol(-1); and (2) a novel experimental membrane manufactured by Ballard Advanced Materials Corporation (BAM3G(TM) 407) with an equivalent weight of 407 g mol(-1). These two membranes differ in their chemical structure, equivalent weight, and water content. Solid-state electrochemistry shows that the permeability (the product of diffusion coefficient and solubility) of oxygen for the two membrane materials is approximately the same. The solubility of oxygen in the novel BAM3G 407 membrane investigated is lower than that for Nafion 117 by a factor of four. The diffusion coefficient of oxygen in BAM3G 407 on the other hand is greater by a factor of four. The difference in mass transport properties is explained on the basis of the much higher water content in the BAM3G 407 membrane (87 wt.% H2O) compared to Nafion 117(19 wt.% H2O).