A transient, generic contamination model addressing kinetic losses in proton exchange membrane fuel cells (PEMFCs) with closed-form solutions is presented. The model is applicable to both the cathode (oxygen reduction reaction) and anode (hydrogen oxidation reaction), because the reactant rate-determining step (rds) in each case is similar and involves an intermediate adsorbate species. The contaminant rds is either a reaction or desorption of an adsorbed product. The model is validated using both fuel cell and sensor operation data lot a range of designs (catalysts including Pt and WC) and operating conditions (CO, SO2, NO/NO2, and H2S introduced in the cathode or anode compartment). Model validation indicates the need for determined experimental procedures, including use of a controlled potential, design of setups avoiding contaminant residence time delays in pipes, data recording with sufficient resolution and over long periods to capture steady states, and duplicate experiments to evaluate reproducibility. Model validation also indicates that the contaminant reaction appears to be the prevalent rds. An approach to unambiguously obtain rate constants is proposed requiring sequential tests performed with a reactant, a contaminant, and a reactant/contaminant mixture. (C) 2008 The Electrochemical Society.