The relationship between the microstructure of platinum-carbon (Pt/C) agglomerates and cathode performance was investigated for membrane-electrode-assemblies (MEAs) with a hydrocarbon-based (HC) binder and a poly (perfluorosulfonic acid) (PFSA) binder. The MEA with an HC binder exhibited a higher gas diffusion resistance than that with the PFSA binder. SEM, TEM, and pore size distribution measurements showed that the HC binder was likely to cover a larger area of the carbon support surface compared with the PFSA binder, and that a large amount of the HC binder easily penetrated the primary pores inside the Pt/C agglomerates, which decreased the volume of the pores. It seems probable that the HC binder in the primary pores blocked the oxygen diffusion to the cathode catalyst. Based on the above consideration, we focused on increasing the primary pore volume. Consequently, the volume was doubled and the gas diffusion resistance at 0.25 A/cm(2) was successfully reduced from 1600 to 410 m Omega.cm(2). (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3604396] All rights reserved.