Hydrogen bleeding into the cathode inlet of a proton exchange membrane (PEM) fuel cell could be a simple approach to reduce the H-2 concentration in the fuel cell exhaust during transient operating conditions (e.g., start-up or fast transients) of a PEM fuel cell system; it could also serve as an additional heating source during cold start-up. In this experimental study, we address the question whether the chemical stability of the polymer electrolyte membrane is affected negatively by a hydrogen bleed into the cathode inlet of a PEM fuel cell. First, rotating ring disc electrode (RRDE) experiments were carried out to detect whether any additional H2O2 is produced during the oxygen reduction reaction in O-2 saturated electrolytes in the absence and presence of H-2. Dry open circuit voltage (OCV) experiments were then performed for more than 250 hours in 50 cm(2) single cells at 120 degrees C and 18% relative humidity (RH) in order to investigate the effect of a 4 vol. % H-2-bleed into the cathode inlet on membrane stability. Finally, the distribution of membrane pin-holes was determined on membrane electrode assemblies (MEAs) after the dry OCV tests conducted with or without H-2-bleed using an infrared (IR) camera setup. In addition, the diffusion-limited hydrogen oxidation current on the cathode side was modeled in order to estimate the maximum areal heat flux near the cathode inlet, which would be caused if the H-2 oxidation rate were to be diffusion-limited. (C) The Author(s) 2017. Published by ECS. All rights reserved.