We investigated the durability of cathode catalysts during a start-up (SU) process similar to that used in actual polymer electrolyte fuel cell vehicle operation. The durability of Pt supported on graphitized carbon black (Pt/GCB) catalysts was evaluated in the practical SU process, i.e., the anode gas was successively cycled between air, hydrogen, and nitrogen. The effect of the SU process on the cell performance was evaluated using two types of catalysts (commercial Pt/GCB, and that prepared in house by the "nanocapsule-method," n-Pt/GCB). The polarization curves and cyclic voltammetry were evaluated before and after the SU evaluation. The degradation of Pt nanoparticles and carbon supports was analyzed by transmission electron microscopy, scanning transmission electron microscopy, and micro-Raman spectroscopy. We also applied glancing incidence X-ray diffraction in order to observe the depth profiles of the Pt crystallite sizes in various interfacial regions. From these analyses, we found that the degradation of the Pt catalyst occurred not only in the gas outlet region but also the gas inlet region of the cathode. The degradation in the inlet region is ascribed to both the interim electrochemical evaluations and the potential fluctuations, which cause a dissolution of Pt nanoparticles used during the SU process. (C) The Author(s) 2016. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly All rights reserved.