When cationic impurities, such as Na+, Ca2+, or metal cations, are present in a polymer electrolyte membrane fuel cell (PEMFC) the performance of the PEMFC can be significantly reduced. These effects were modeled in a PEM by solving the Nernst-Planck equation for all cationic species in the membrane. The model shows that cationic contaminants will always be more concentrated on the cathode side of the PEM when current is drawn through the PEM. This was attributed to the trade off between diffusion and migration of contaminants in the membrane. It was then theorized that at high currents and contamination levels a maximum current density of protons through the PEM would be encountered. This model allows us to better understand the effects of cationic contamination on PEMFCs. This understanding should lead to development of new modes for diagnosis and better methods to recover fuel cell performance. (C) 2008 Published by Elsevier Ltd.