We characterized the effects of cobalt (Co2+) and other cation contaminants on the oxygen (O-2) transport properties of the PFSA ionomer used in polymer electrolyte membrane fuel cells (PEMFCs) and gained insight into the mechanisms by which contaminant cations inhibit O-2 transport. Such cations can be released by alloy catalysts and environmental conditions and pose a significant challenge to maintaining high current density performance with low platinum (Pt) loadings. We used a test cell capable of isolating the ionomer from the membrane electrode assembly (MEA), allowing for O-2 transport resistance (RO2) measurements using a limiting current technique. We contaminated ionomer membranes with Li+, Na+, Ni2+, Co2+, and Ce3+ and found a general increase in RO2 for increased contamination levels and decreased water activity. In addition, our Co2+ results indicated distinct concentrationdependent regimes. The other cation-form ionomers allowed us to separate the impacts of ion pair strength, multivalency, and reduced water uptake. We believe that these factors cause a more compressed hydrophilic domain and tortuous O-2 diffusion path, and a commensurate increase in RO2. Finally, we studied the impact of Co2+ on an operating PEMFC and found an increase in RO2 consistent with the results of our isolated membrane tests. (C) 2019 The Electrochemical Society.