Cross-sectional visualizations of catalyst-coated membranes (CCMs) with various weight ratios of Nafion (R) ionomer to carbon (N/C) in catalyst layers (CLs) of a proton-exchange membrane fuel cell were performed by a cross-section polishing method and scanning electron microscopy. Nitrogen physisorption measurements and energy-dispersive X-ray spectroscopy studies were also performed. The CL structure clearly affected the performance of the cell. Sufficient proton, electron, and oxygen paths are probably established in N/C 1.0, resulting in relatively higher cell performance per unit of Pt loading, whereas in N/C 3.0, insufficient transport paths were produced as a result of plugging of nanoscale pores by Nafion (R) ionomer. However, the effective gas diffusivity in N/C 0.3 may be increased because of increases in porosity and pore diameter. Methods for controlling the macroscopic structural parameters of CLs during their manufacture are discussed; the thickness and porosity are determined mainly by the carbon and the ionomer, respectively. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.