Mercury porosimetry, scanning electron microscopy, and transmission electron microscopy have been used to characterize the pore size and distribution of carbon supports, with and without catalyst loading, and the catalyst layer of membrane electrolyte assemblies (MEAs) used in polymer electrolyte fuel cell (PEFC) applications. The majority of pores for carbon powders and catalyst loaded carbon powders concentrate in a range from 24 to 84 nm (in diam) with a small number of residual pores lying outside this range. The membrane electrode assembly catalyst layer prepared from recast Nafion and carbon supported catalyst has a different pore structure, determined by the type of catalyst used, the preparation method, and the total Nation content. The catalyst layer has two distinctive pore categories with boundary of similar to17 nm in diam. The specific pore volume of both the primary (small) pores and the secondary (large) pores decreases with an increase of Nafion content. Boiling the MEA tremendously increases the pore area. The Nafion content in the catalyst layer affects the performance of MEAs over the entire current range. The optimum performance was obtained with a Nafion content of 28 wt % for a Pt3Cr catalyst layer made by the thin film decal process. (C) 2004 The Electrochemical Society.