There are significant impacts to fuel cell performance when contaminant cations replace protons in the ionomer. The purpose of this paper is to directly correlate extent of cationic contamination with fuel cell performance and also to elucidate main mechanisms responsible for this performance degradation. A series of standard membrane electrode assemblies (MEAs) were contaminated by varying the ratio of protons to cesium cations in the ionomer phase. The amount of contamination was determined using X-ray Fluorescence (XRF) Spectroscopy. These MEAs were then run in fuel cell mode to determine fuel cell performance as a function of cationic contamination level. A strip cell configuration was also utilized to distinguish between ohmic, thermodynamic, and kinetic effects. Additionally, the distribution of contaminants was measured using XRF. The decrease in fuel cell limiting current, power density, and membrane conductivity as a function of contamination level were quantified. Performance decreased monotonically with increased contamination. The cationic contaminant was preferentially located near the cathode of the strip cell under load. This distribution led to increases in thermodynamic and kinetic polarizations that were more important than changes in membrane conductivity. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3610986] All rights reserved.