Hydrogen permeation across the membrane is unavoidable in proton exchange membrane fuel cells, especially for super-thin membranes, which lowers the open-circuit voltage and could even be a safety concern. In this paper, hydrogen permeation across two membranes (25-um-thick Nafion (R) 211 and 18-um-thick reinforced composite membrane) are evaluated at various temperatures, relative humidity (RH), and gas pressure differences between the anode and cathode. The results indicate that the hydrogen permeation rate in both membranes increases almost exponentially with temperature and linearly with pressure differences. Compared with RH, the effects of temperature and pressure differences are more crucial to hydrogen permeability. However, the effect of RH on the hydrogen permeation is quite complicated. The permeability exhibits a minimum value at intermediate RH (approximately 40% RH) for both applied membranes. The permeability of Nafion (R) 211 appears more sensitive to RH than that of reinforced composite membrane at elevated temperature. Copyright (C) 2013 John Wiley & Sons, Ltd.