Start-up and shut-down cycling in PECFs causes severe degradation, because the carbon supports at the cathode catalyst layers are corroded due to the instantaneous reaction with H2O. We developed a new CO2 visualization system using a CO2-sensitive dye for detecting the generation of CO2 during the corrosion processes in real time and with spatial resolution. The corrosion occurring during the shut-down process continued longer near the inlet and produced a more serious effect than during the start-up process, near the outlet. This new CO2 visualization technique should help to minimize the corrosion of carbon supports in all types of PEFCs. (C) 2012 The Electrochemical Society. [DOI:10.1149/2.018204esl] All rights reserved.