Polymer electrolyte membrane water electrolysers (PEMWEs) are a promising technology for hydrogen production but catalyst degradation mechanisms are poorly understood, hampering informed catalyst design for extended lifetimes and the use of more economical loadings. Here we demonstrate the application of an innovative reference electrode to the study of catalyst degradation in an operating PEMWE. This approach enables separation of the relative contribution of anode and cathode to the overall water splitting reaction. It is shown that, in shut-down periods during power cycling, the cathode contributes more to changes in open circuit voltage than the anode. Changes in the electrochemical surface area of the platinum cathode as a result of power cycling are measured in situ for the first time using hydrogen underpotential cyclic voltammetry. The results demonstrate that degradation of the platinum cathode plays a more significant role than conventionally assumed by the electrolyser community, which has tended to focus more on the iridium/ruthenium oxide anode because it dominates the performance of the cell.