Progression of a fuel/air front through the anode flow-field during start-up or shut-down of a polymer electrolyte membrane fuel cell is known to generate elevated cathode potentials, leading to corrosion of the carbon catalyst support. Here we present spatially resolved measurements of such potential transients in an operating fuel cell, using an innovative reference electrode array combined with quantification of carbon corrosion by measurement of CO2 in the cathode outlet. A systematic study of the effect of relative humidity on start-up/shut-down potential transients and carbon corrosion rates was carried out at open circuit and with the application of a small external load. The results are discussed in the context of a schematic framework for the reverse current decay mechanism expressed in terms of local electrode potential. In all cases carbon corrosion was more severe during start-up than during shut-down, with the highest cathode potentials measured opposite the anode outlet during start-up and opposite the anode inlet during shut-down. The carbon corrosion rate was least severe under the driest conditions, which was attributed to the increased membrane resistivity. This new technique provides a powerful diagnostic tool for evaluation of start-up/shut-down tolerant catalyst layers and optimisation of fuel cell hardware design.