Chemical membrane degradation causes deterioration of critical membrane properties such as gas separation which finally causes failure of polymer electrolyte membrane fuel cells (PEMFCs). In order to identify the underlying physical processes, a physics-based model of chemical membrane degradation is implemented into the novel numerical framework NEOPARD-X [1]. The existing 2D PEMFC model is extended to incorporate the mechanisms of hydrogen peroxide formation and reduction, a redox cycle of iron contaminants in the ionomer phase, radical formation due to Fenton's chemistry and radical attack on the polymer structure. Unzipping of the polymer backbone and scission of the side chains are considered as degradation mechanism. The degradation model is validated against experimental data obtained in accelerated stress tests (ASTs). From theoretical considerations, the influence of chemical membrane degradation on the cell performance is revealed. The influence of pressure, relative humidity and cell voltage on the chemical degradation is rationalized. The operating conditions strongly influence the kinetics and spacial distribution of the membrane degradation. Degradation is found to be most pronounced at elevated pressure, high relative humidity and high cell voltage close to the interface of anode catalyst layer and PEM.