In this study, the structural degradation of a polymer electrolyte membrane fuel cell (PEMFC) cathode catalyst layer due to carbon corrosion was investigated. To oxidize the catalyst carbon support, the PEMFC catalyst layer was subjected to a 30 h accelerated stress test that cycled the cathode potential from 0.1 to 1.5 V-RHE (where RHE denotes reversible hydrogen electrode) at 30 and 150 s intervals. Carbon dioxide release was measured in the gas exhaust to establish the rate and amount of carbon loss. Cyclic voltammetry, electrochemical impedance spectroscopy (EIS), scanning electron microscopy, and polarization analyses were completed to characterize and correlate the structural degradation of the catalyst layer to the PEMFC performance. The results showed a clear thinning of the cathode catalyst layer and the gas diffusion layer carbon sublayer and a reduction in the effective platinum surface area due to carbon support oxidation. The degradation of the cathode catalyst layer also altered the water management, as evidenced by an increase in the voltage losses associated with oxygen mass transport and catalyst layer ohmic resistance. There was an emphasis on the EIS measurement to further develop and verify this methodology for other degradation mechanisms.