This study presents an improved 1-D PEM fuel cell model, which includes a new submodel for interface mass transfer in the presence of evaporation and condensation. In contrast with the conventional approach, this new submodel describes mass transfer under equilibrium conditions without the need for empirical coefficients. Polarization curves calculated with the improved model agree with experimental data from the literature. Simulation results illustrate the importance of evaporation and condensation in PEM fuel cells. The submodel developed here can also be used for heat and water management in PEMFC design.