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 procedure leads to the best fuel cell performance compared to fuel cells which are used after a rapid break-in or rather without any break-in procedure. A combination of synchrotron X ray radiographic measurements with in-situ performed time-dependent impedance spectroscopy was used to investigate the de-/hydration processes within the electrolyte of the membrane electrode assembly (MEA) components of a dynamically operated HT-PEFC. This helps to understand the fuel cell performance under different operating conditions.