The oxygen reduction reaction on platinum interfaced with phosphoric acid doped PBI at elevated temperature and low relative humidities has been investigated by using a micro band electrode technique. Both the kinetic and the mass transport parameters in the Pt/PBI-H3PO4 system are comparable to those of the Pt/H3PO4 system under similar conditions. The study suggests that it is the amorphous H3PO4 phase that functions as the electrolyte. The oxygen reduction reaction is first order with respect to both proton concentration and oxygen saturation concentration in the electrolyte, which indicates that the proton transfer is the rate-determining step in oxygen reduction. The H3PO4 doping level and the water content of the electrolyte affect the ORR exchange current density, oxygen diffusion, and the oxygen solubility in PBI-H3PO4 membranes. The dissolved O-2 molecules permeate mainly through the amorphous H3PO4. However, the oxygen solubility in PBI-H3PO4 is higher than its solubility in H3PO4, which is explained by the presence of the crystalline PBI region formed during electrolyte preparation. (c) 2005 Elsevier Ltd. All rights reserved.