The involvement of H2O2 in the membrane degradation mechanism in a polymer electrolyte membrane fuel cell (PEMFC) was investigated. Measurement of fluoride concentration in the effluent water was used as an indicator of the membrane degradation rate. It was found that H2O2 is formed in the fuel cell in small concentrations but is not the main source of harmful species, which degrade the membrane. H2O2 decomposition due to impurities or the catalyst leading to the possible formation of radical species would only account for a small fraction of the membrane degradation rate in a fuel cell.