A two-phase model of an intermediate temperature (120-200 degrees C) proton exchange membrane (PEM) fuel cell is presented. This model is an improvement on our previous intermediate temperature models, in that it accounts for two phase effects due to gas solubility in the phosphoric acid/polybenzimidazole (PBI) electrolyte, and considers aqueous phase electrochemical reactions. The model accounts for all polarization and transport phenomena, and exhibits a good fit with experimental data in the temperature range (150-170 degrees C). Parametric results are also presented, to investigate the dependence of the fuel cell performance on membrane doping level, catalyst activity, and transport properties of dissolved gases in the electrolyte medium. Results indicate that significant transport limitations exist at both electrodes, where catalyst utilization is between 0.1% and 1%. Alternative doping agents to phosphoric acid are suggested, as well as the need for more advanced catalyst deposition techniques. (c) 2006 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.