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Pinhole formation in proton exchange membranes (PEM) due to hygrothermal stresses during the operation of a fuel cell can be interpreted as a result of crack formation and propagation. Fracture mechanics is one possible approach to characterizing failure. The goal of this study is to measure the intrinsic fracture energy of various commercially available PEMs. The intrinsic fracture energy has been used to characterize the fracture resistance of polymeric materials with minimal plastic dissipation and the absence of viscous dissipation, and has been associated with the long term durability of polymeric materials where subcritical crack growth occurs under slow time-dependent or cyclic loading conditions. Insights into this limiting value of fracture resistance may offer insights into the durability of PEMs. A knife slit test was conducted to measure the fracture energies close to intrinsic fracture [1].