Abstract High Temperature Polymer Electrolyte Membrane (HT-PEM) fuel cells are a promising choice for renewable power generation. We present an experimental study that investigates the durability of a full-size fuel cell stack, where membrane electrode assemblies were continuously extracted during operation of the stack, and analysed in detail with electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction. The evolution of membrane thinning, the loss of phosphoric acid and the changes in the catalyst layer of the electrodes have been investigated as a function of operation time. A correlation between the acid loss and the decrease of cell voltage has been determined to provide detailed insight into durability data of the HT-PEM fuel cell on a commercial sized stack level.