This study is focused on elucidating the catalytic combustion phenomenon in proton-exchange membrane fuel cells. A visualization cell and an infrared (IR) camera are used to capture the thermal behavior under combined chemical and mechanical accelerated stress conditions in situ. A 3.4 cm 2 catalyst coated membrane (CCM) embedded in the cell is subjected to a relative humidity (RH) cycling test at atmospheric pressure and a cell temperature of 80 °C under open-circuit voltage (OCV) conditions. The temperature distribution on the gas diffusion layer surface at the cathode is captured using a high-transmittance glass window (ZnS window). Continuous IR imaging revealed a hot spot at ca. 500 RH cycles, suggesting the existence of a pinhole in the degraded CCM and the occurrence of catalytic combustion there. The occurrence of the hot spot coincides with the time at which the electrochemical indicators detect membrane failure, i.e., hydrogen crossover rate, OCV. Furthermore, the post mortem analysis revealed a 105-micrometer diameter pinhole, the position of which matched that of the hotspot. An accidental combustion during the durability test could cause the formation of a pinhole. This pinhole is responsible for the rapid increase in the hydrogen crossover rate as well as the significant decrease in the OCV at 500 RH cycles until the end of the durability test. Figure 1