Triple Phase Boundary (TPB) properties at Pt/Nafion interfaces in a Polymer Electrolyte Membrane (PEM) fuel cell depend on the relative rates of reaction and diffusion processes. In this paper, electrochemical impedance analysis is performed to model the coupling between the reaction and diffusion phenomena in the TPB zone. The transfer function for the PEM fuel cell system is obtained for the oxygen reduction reaction (ORR) mechanism with two adsorbed species. An analytical expression is derived for the TPB diffusion impedance element based on the transient concentration profile and the ORR kinetics derived for Pt microelectrodes placed on Nafion electrolyte. For this arrangement, the coupled kinetic-diffusion impedance is shown to have a distinct gain and phase relationships, which is closely related to the Gerischer element. The model is used to study the transient response of the coupled impedance element in terms of TPB parameters and operating voltages. The TPB impedance element, in conjunction with existing models of mass transport impedance in GDL and porous electrodes, provides a more comprehensive analysis of combined Faradaic and diffusion processes occurring at the Pt/Nafion interfaces in PEM fuel cells. (C) 2011 The Electrochemical Society. [DOI: 10.1149/1.3596020] All rights reserved.