Chemical membrane degradation is a major limiting factor for polymer electrolyte fuel cell (PEFC) durability and lifetime. While the main degradation mechanisms are established in the literature, the in-situ trends of their action are often only known qualitatively. This motivates the development of a comprehensive in-situ chemical membrane degradation model addressed in this work. The numerical algorithms developed are strategically designed to be compatible with state-of-the-art computational membrane electrolyte assembly (MEA) performance models; here, we emphasize the integration of the developed degradation model into a 1-D MEA transport-reaction model to determine the linkages between the MEA macroscopic phenomena, in-situ operating conditions, and the temporal membrane degradation process. Concentrations of hydrogen peroxide, radical, and degraded ionomer species are, modeled to interrogate the evolution of ionomer molecular structure with respect to the chemical membrane degradation. The proposed degradation mechanism includes the initiation and propagation of side chain degradation culminating in main chain scission and fragmentation, and demonstrates a good agreement with the most recent experimental findings. The integrated MEA model is further applied to simulate the macroscopic effects of chemical membrane degradation characterized in a recent accelerated stress test. Comparisons between the numerical and experimental results are discussed. (C) 2014 The Electrochemical Society. All rights reserved.