Differently conditioned high temperature polymer electrolyte fuel cells (HT-PEFCs) were investigated by electrochemical impedance spectroscopy to study the influence of the break-in procedure on the overall fuel cell performance during the heating-up procedure as well as during different operating conditions. It is shown that a classical 70 h break-in leads to better fuel cell performance compared to a fuel cell which is used without any break-in procedure. A combination of synchrotron X-ray radiographic measurements with in situ time-dependent impedance spectroscopy was used to investigate the de-/hydration processes of the phosphoric acid type electrolyte within the components of the membrane electrode assembly (MEA) of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions. (C) 2012 The Electrochemical Society. [DOI: 10.1149/2.024208jes]