Achieving practical stability and durability of PGM-free catalysts, e.g., M-N-C catalysts (where M, N, and C represent a transition metal, nitrogen, and carbon, respectively), remain a great challenge for the practical application of these catalysts in polymer electrolyte fuel cell (PEFC) cathodes. A variety of degradation paths have been proposed for M-N-C catalysts, including irreversible dissolution of the M from the active site and corrosion of the C matrix.1 Both mechanisms are active areas of research but it remains difficult to de-convolute contributions from each and how these mechanisms couple to environmental conditions (temperature, pH, fuel cell humidification, reactant flow rates, etc.). It is possible that metal dissolution occurs which destabilizes the local C which is then more easily corroded. It is also possible that C corrosion occurs which destabilizes the local M binding environment leading to dissolution of M. It is further possible that free radicals generated from 2-electron ORR pathways (either on M sites or defected-C sites) subsequently attack the C support, the M, or both. Also, the mechanisms may not be directly linked. Thus, without further understanding of each mechanism, it is difficult to prescribe methods of material stabilization or even accelerated stress tests for exacerbating a given activity loss mechanism.In this talk, combined experimental and theoretical C corrosion studies of M-N-C catalysts will be presented. This work includes operando quantification of CO2 emission by nondispersive infrared detection and inductively coupled plasma measurement of metal dissolution as a function of fuel cell conditions. Specifically, the C corrosion of M-N-C catalysts will be evaluated at different relative humidity, cell temperature and reactant stoichiometry. M-N-C catalysts with various C structure will also be compared for exploring the possible mechanism of C corrosion in PGM-free catalysts. DFT calculations of C corrosion and oxidation as a function of atomic scale structure will also be discussed for comparison with metal dissolution stability studies. Overall, we aim to decouple de-metalation and C corrosion mechanisms for activity loss in PGM-free ORR electrocatalysts in order to better mitigate such losses, thus strengthening their case as PGM replacements in fuel cell applications.