Four hundred hour fuel cell tests were performed on commercial, as-received and Ca2+ contaminated catalyst coated membranes (CCMs) to evaluate the effects of long term exposure of foreign cations on fuel cell performance and degradation. Following testing, significant thinning of the cathode catalyst layer was observed across the entire active area in the contaminated cell, while only localized thinning was observed in the as-received baseline CCM. Analysis of the elemental maps and line intensity profiles of platinum (Pt) obtained from energy dispersive X-ray spectrum (EDX) shows minimal change in total platinum content across the thickness of the catalyst layer, and hence loss of carbon is suspected to be the cause of the thinning. A numerical model is employed to show the foreign cation redistribution during testing, which shows proton depletion in the cathode catalyst layer when the CCM is contaminated with foreign cations. We hypothesize that this lack of protons leads to accelerated carbon corrosion in the cathode to supply protons to oxygen reduction where foreign cations block transport of protons. A carbon corrosion scheme is presented which shows that under the operating potentials of the cell, carbon oxidation can occur, leading to the observed thinning. (C) The Author(s) 2018. Published by ECS.