To correlate the cationic contaminant uptake of the catalyst coated membrane (CCM) with the polymer electrolyte fuel cell (PEFC) performance, CCMs were contaminated by saturating in varying concentrations of H2SO4 and Al-2(SO4)(3) solution, establishing ex-situ equilibrium cation uptake. The CCMs were then assembled and tested in single cell hardware to determine the performance as a function of cationic contamination level. Smaller CCM coupons were also saturated at the same time as the full-size CCM and were characterized by acid site titration to determine the cationic occupation fraction. It is found that the acid site occupation in the CCM increased and the cell performance decreased with increased Al ion concentration in the solution. With less than 10% occupation, the cell performance dropped from 1000 mA/cm(2) to 100 mA/cm(2) at 0.6 V. Upon re-protonation of the CCM with H2SO4, it is found the cell performance was fully recovered. (C) 2013 The Electrochemical Society. All rights reserved.