A mathematical model is developed which describes the nature, properties, and scaling of the triple phase boundary (TPB) for a Pt/Nafion polymer electrolyte membrane fuel cell (PEMFC) system. The model incorporates coupled reaction and diffusion phenomena, leading to a concept of the TPB not as a singularity, but rather as having an "effective width." The "effective width" of the TPB depends on the interplay between the relative rates of the reaction and diffusion processes at the PL/Nafion interface. Implications of the model for PEMFC catalyst layer design are explored. Additionally, scaling predictions of the model are compared with kinetic observations from geometrically well-defined Pt-microelectrode/Nafion experiments and are shown to match favorably with experimental results. (C) 2005 The Electrochemical Society. All rights reserved.