The performance of polymer electrolyte fuel cells degrades rapidly when cells are exposed to synchrotron radiation (SR) in e.g. X-ray tomographic microscopy (XTM) experiments. Yet, detailed mechanisms of the damage are unknown. Here, the interaction of the cell components with SR is investigated in terms of mechanical and chemical properties as function of the absorbed energy at a photon energy of 13.5 keV with a flux density in the order of 10(13) photons s(-1) cm(-2). Ex-situ experiments with membranes, catalyst coated membranes, gas diffusion layers (GDL), catalyst coated GDL and PTFE foil are compared to the in-situ degradation dynamics. The external advancing and receding contact angle between water and the substrate (GDL and PTFE), investigated with a Wilhelmy method, decrease with increasing SR exposure. The stress-strain characteristics, the equivalent weight and the FTIR analysis of the membrane indicate that the main chain of the ionomer is cleaved. It is hypothesized that the main reason for the performance degradation under X-ray irradiation is caused by a loss of electrochemically active catalyst surface area due to ionomer degradation and a resulting disintegration of the catalyst layer. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.042208jes]