A gas diffusion layer (GDL) was subjected to accelerated degradation (ageing) via complete submersion in hydrogen peroxide. The impacts of ageing were evaluated through deviations in limiting current densities, oxygen mass transport resistances, and liquid water saturation distributions in the aged GDL compared to pristine materials. For dry (volumetric oxygen concentration of 1%) and partially saturated cathode conditions (volumetric oxygen concentrations of 5%, 10%, and 21%), the fuel cell composed of aged GDLs reached limiting current densities up to 10.3% lower and experienced increases in overall oxygen mass transport resistance of up to 11.5% compared to the pristine GDLs. This performance reduction was attributed to a higher liquid water content in the aged GDL under the flow field channel compared to the pristine GDL. The results suggest that the higher saturation in the channel regions of the aged GDL led to increasingly restrictive gas transport due to the higher average water content and reduced pathways for reactant gas transport, therefore contributing to the reductions in the limiting current. In fuel cell builds with aged and pristine GDLs, up to 13.6 times more liquid water was present in the GDL under the rib compared to that under the channel at limiting current operation. (C) 2017 The Electrochemical Society. All rights reserved.