Durability is a major limitation of current proton exchange membrane fuel cells. Mechanical stress due to hygro-thermal cycling is one failure mechanism of the polymer electrolyte membrane (PEM). In a fuel cell the PEM is highly constrained in the membrane plane and relatively unconstrained in the through-thickness direction, leading to primarily biaxial loading upon hygro-thermal cycling. The rate, temperature, and hydration dependent elastic-viscoplastic mechanical behavior of Nafion, the benchmark PEM, has been extensively investigated in uniaxial tension in prior work which also served as a data basis for a three dimensional constitutive model. Here, the important effects of the biaxiality of the loading conditions on the elastic-viscoplastic Nation stress strain behavior are investigated for the first time via experiments and simulation. Biaxial stress strain behaviors were shown to exhibit similar features to uniaxial behavior including linear elasticity followed by a highly non-linear transition to yield followed by post-yield strain hardening with highly non-linear unloading and reloading: these features were each quantitatively dependent on the biaxiality of the loading conditions. The constitutive model was found to successfully quantitatively predict the loading behavior and its dependence on biaxiality. The constitutive model was also found to predict the magnitude of the yield shoulder during unloading and reloading, but to underestimate the gradual nature of both the forward and reverse plastic deformation processes as well as the strain recovery at zero load. These errors are consistent with those seen in the uniaxial model indicating that the framework used to incorporate the uniaxial behavior into a three dimensional model is capable of predicting the biaxial deformation response of the membrane. (C) 2010 Elsevier Ltd. All rights reserved.