When cationic impurities, such as Na+, Ca2+, ammonium or metal cations, are present in a polymer electrolyte membrane fuel cell (PEMFC) the performance of the PEMFC can be significantly reduced. A half cell model was developed to explain the most relevant processes causing performance degradation in fuel cells under cationic contamination. In fuel cells contaminated with foreign cations the exchange current density and the limiting current decreases. These effects can be explained by accounting for the concentration of available protons in the CCL and by the transport of cations by diffusion and migration.