Extensive corrosion has been shown to result in collapse of the porous cathode microstructure in PEMFCs and has been presumed responsible for severe mass-transport voltage losses observed in polarization data. Corrosion of the carbon support was studied by electrochemical methods, non-dispersive infrared spectroscopy, and FIB-SEM tomography to evaluate the connected roles of carbon loss, microstructure, and performance degradation. In silico characterization of the three-dimensional, digital microstructures generated by FIB-SEM included porosity, Dijkstra tortuosity, and two-point correlation functions before and after corrosion. Carbon loss and H-2/air operando performance decay were strongly correlated but contrary to expectation, the associated porosity loss and tortuosity gain did not explain the observed performance decay; cell failure occurred long before microstructural collapse was observed. Platinum catalyst ripening and preferential corrosion of the amorphous interior of carbon black primary particles were likely the two predominant degradation mechanisms induced among the cells tested. These results connect and clarify the roles of performance, microstructure, and corrosion degradation and demonstrate that microstructural collapse need not underlie cell failure. (C) 2017 The Electrochemical Society. All rights reserved.