Water and heat are produced in the cathode catalyst layer of a polymer electrolyte fuel cell (PEFC) due to the oxygen-reduction reaction. Efficient water removal from the gas diffusion layer (GDL) to the flow channel is critical to achieve high and stable PEFC performance. Water transport and removal strongly depend on local temperature because the saturation concentration of water vapor rises rapidly with temperature, particularly in the temperature range of practical interest to PEFC applications. Detailed investigations of two-phase flow in the GDL have been reported in the literature, but not on the rate of phase change - either from liquid to vapor as in the case of evaporation or from vapor to liquid as in the case of condensation. In the present work, a two-phase, non-isothermal numerical model is used to elucidate the phase-change rate inside the cathode GDL of a PEFC. Results computed from our model enable a basic understanding of the phase-change processes occurring in a PEFC.