A new method of evaluating gas transport resistance inside and outside the carbon supports of fuel cell catalyst layers has been developed using a partial CO covering technique. This method was developed to examine the effectiveness of the Pt catalyst deposited inside and outside the primary pores of the carbon supports. Gas transport resistance for Pt located outside a carbon support was measured by selectively deactivating Pt inside the support with CO. Gas transport resistance for Pt inside the carbon support was estimated from the total gas transport resistance and that for Pt outside the support. Gas transport resistance for Pt inside the carbon support per unit Pt surface area, r(micro,in,) was slightly greater than that for Pt outside the support, r(micro,out.) This indicates that Pt inside carbon supports is not as effective as that outside the supports. Transport resistance inside the carbon support was investigated in more detail by controlling the degree of CO covering by varying the RH condition for CO oxidation. Gas transport resistance for Pt that became active at higher RH was higher than that for Pt that became active at lower RH. This indicates that gas transport resistance inside carbon supports strongly depends on the pore geometry. (C) 2013 Elsevier B.V. All rights reserved.