Fuel cells are promising candidates for cleaner and more efficient energy conversion but are hampered by cost and durability issues. In order to meet commercialisation targets, there is an urgent need for improved in situ measurement techniques to further understanding of fuel cell degradation mechanisms and facilitate design optimisation. Here we describe an innovative fuel cell reference electrode array using a through-plate configuration, which makes possible for the first time accurate mapping of the variation in electrode potential across the active area of an operating polymer electrolyte membrane fuel cell (PEMFC). This configuration overcomes the major limitation of conventional fuel cell reference electrodes in that potential drop effects are negated. The utility of the technique is neatly demonstrated in the study of catalyst support corrosion during fuel cell start-up, which is associated with movement of the air/fuel boundary through the anode flowfield.