In this paper, hydrogen crossover was measured in an environment of high-temperature proton exchange membrane (PEM) fuel cells using a steady-state electrochemical method at various temperatures (T) (80-120 degrees C), backpressures (P) (1.0-3.0atm), and relative humidities (RH) (25-100%). An H-2 crossover model based on an MEA consisting of five layers - anode gas diffusion layer, anode catalyst layer, proton exchange membrane (Nation 112 or Nation 117), cathode catalyst layer, and cathode gas diffusion layer - was constructed to obtain an expression for H-2 permeability coefficients as a function of measured H-2 crossover rates and controlled H-2 partial pressures. The model analysis suggests that the dominant factor in the overall H-2 crossover is the step of H-2 diffusing through the PEM. The H-2 permeability coefficients as a function of T, P, and RH obtained in this study show that the increases in both T and P could increase the H-2 permeability coefficient at any given RH. However, the effect of RH on the permeability coefficient seems to be more complicated. The T effect is much larger than that of P and RH. Through experimental data simulation an equation was obtained to describe the T dependencies of the H-2 permeability coefficient, based on which other parameters such as maximum permeability coefficients and activation energies for H-2 crossover through both Nation 112 and 117 membranes were also evaluated. Both Nation 112 and Nation 117 showed similar values of such parameters, suggesting that membrane thickness does not play a significant role in the H, crossover mechanism. (C) 2007 Elsevier B.V. All rights reserved.