A one-dimensional non-isothermal model of a proton exchange membrane (PBM) fuel cell has been developed to investigate the effect of, various design and operating conditions on the cell performance, thermal response and water management, and to understand the underlying mechanism. The model includes variable membrane hydration, ternary gas mixtures for both reactant streams, phase change of water in the electrodes with unsaturated reactant gas streams, and the energy equation for the temperature distribution across the cell. It is found that temperature distribution within the PEM fuel cell is affected by water phase change in the electrodes, especially for unsaturated reactant streams. Larger peak temperatures occur within the cell at lower cell operating temperatures and for partially humidifed reactants as a result of increased membrane resistance arising from reduced membrane hydration. The non-uniform temperature rise can be significant for fuel cell stacks. Operation on reformed fuels results in a decrease in cell performance largely due to reduced membrane hydration, which is also responsible for reduced performance at high current densities for high cell operating pressures. Model predictions compare well with known experimental results.