The main long-term purpose of this investigation is a profound understanding of the degradation mechanisms and the identification of fuel cell operating conditions and polymer electrolyte membrane properties which warrant a long lifetime and more reliable behaviour in general. The work involves the development and application of electron paramagnetic resonance (EPR) electrochemistry techniques to study polymer proton exchange membrane degradation in fuel cell applications. As a first step, a miniature cell that can work in a cavity of an X-band EPR spectrometer was built. This is the first report of an in situ fuel cell in an EPR spectrometer. We plan to identify different orgamc radicals which form in the running cell as intermediates of the degradation reactions. The objects of our research are different types of ionically, covalently, and covalent-ionically cross-linked polyaryl-blend membranes.It is demonstrated that tuning of the microwave cavity with a running fuel cell inside is possible, and a pronounced single-line EPR signal is observed. The signal has a g-value of 2.0030, thus it probably originates from the electrode material that is 0.00-2.00 mg/cm(2) Pt on Vulcan XC-72 E-Tek. Its behaviour depends strongly on the fuel cell conditions. It is interesting to note that when the cell is fed with oxygen and hydrogen, the signal intensity diminishes considerably, both under open circuit and under closed circuit conditions. This phenomenon is observed for different membrane types. When the gases are switched off the signal intensity is restored with a characteristic time of 1-2 h. For different membranes, this time differs. In general, water plays a very important role in the observation of EPR-spectra from the in situ fuel cell. (C) 2003 Elsevier B.V. All rights reserved.