Kinetic and mass-transport properties were investigated for the oxygen reduction reaction for Nafion((R)) 117 and sulfonated poly(arylene ether sulfone) membranes, both pre- and post-sulfonated analogs (SPES-40 & SPES-PS) under 100% relative humidity and as a function of pressure (1-4 atm total pressure, 323 K) and temperature (303-353 K, 3 atm) using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass-transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)\proton exchange membrane (PEM) interface. The oxygen reduction kinetics were found to be similar for all the membranes at the Pt microelectrode interface. The temperature dependence of O-2 permeation parameters showed identical trends for the membranes studied while the pressure dependence of O-2 permeation parameters displayed some differences. Despite higher ion exchange capacities and hence higher water uptake, the two SPES membranes exhibited relatively lower values of D as compared to Nafion((R)) 117. The results are discussed in the context of their different microstructures. (C) 2004 Published by Elsevier B.V.