In this study the cause and effect of fuel electrode carbon corrosion in high temperature polymer electrolyte fuel cells are highlighted for the first time. Here, measurements of the CO2 concentration in the fuel electrode effluent and spatially resolved current mapping suggest that the reverse-current decay mechanism, which is responsible for the well-known air electrode carbon corrosion, is not responsible for the observed fuel electrode carbon corrosion. We propose a chemical carbon oxidation mechanism, caused by the alternating exposure of the fuel electrode to hydrogen and air during start-up/shut-down cycling. Furthermore, a degradation study was performed which indicates a severe performance decrease, up to 40 %, due to fuel electrode carbon corrosion after 600 start-up/shut-down events under reformate conditions.