A new method was developed for evaluating the gas transport resistance of the catalyst layer, R-CL, based on gas crossover through the Nafion (R) membrane. Using this method, gas transport resistance toward Pt catalyst (per unit of Pt surface area), R-micro,R-Pt, was investigated for both hydrogen and oxygen. This method is applicable to low relative humidity (RH) conditions because of the extremely low limiting current density attributed to crossover gases. It was used to investigate the RH dependence of R-CL for hydrogen and oxygen, which has so far been difficult to measure. A comparison of the limiting current density due to the hydrogen oxidation reaction (HOR) with and without inert gas flow suggested that the decrease in the HOR current at a high potential is mainly due to mass transfer loss caused by a loss of effective surface area, S-eff, and not by a loss of inherent Pt activity. For hydrogen, it is possible with this method to evaluate the potential dependence of S-eff for the electrochemical reactions rather than the electrochemical surface area (ECA) determined by proton adsorption in the low potential region. It was found that R-micro,R-Pt increased for both hydrogen and oxygen with a lower RH. The trend of RH dependence and the impact of RH on R-micro,R-Pt were very similar to the tendencies observed for the Nafion (R) membrane. However, the gas transport resistance of the ionomer was notably higher for oxygen than for hydrogen, considering its relationship in the membrane. Adsorption of the ionomer onto the Pt/C catalyst might be one of the factors accounting for this difference. (c) 2013 Elsevier B.V. All rights reserved.