We present the first high-resolution synchrotron X-ray study on high temperature polymer electrolyte fuel cells (HT-PEFCs) in through-plane mode. Distribution and evolution of the phosphoric acid in the membrane electrode assembly was monitored in situ/in-operando under different operating conditions at steady states. At current densities of 350 mA cm(-2) and 600 mA cm(-2) significant changes in local media distribution were detected mainly beneath the channels of the flow field. We assign this effect to a varying degree of acid doping over the active region. Furthermore it was found that parts of the electrode structure were moved and partly irreversibly displaced after cell operation at load conditions. This effect might contribute to structural aging of the electrodes. (C) 2013 Elsevier B.V. All rights reserved.