Diagnostics of performance degradation is important for improving the durability of proton-exchange membrane (PEM) fuel cells. X-ray computed tomography (CT) is a valuable non-destructive method to study the internal structure of PEM fuel cells. However, sample cutting is usually needed for high-resolution observations, which prevents the observer from obtaining information about morphology changes during fuel cell testing. In this study, oblique soft X-ray CT has been developed and its suitability as a non-destructive method for PEM fuel cell diagnostics without sample cutting is demonstrated. The CT images of a membrane-electrode assembly (MEA) were obtained at several stages of cell operation, including hot-pressing, load cycles, wet/dry cycles, and start-up/shutdown (SU/SD) experiments. After SU/SD operation, carbon corrosion with newly generated cracks was observed in the catalyst layer at the cathode outlet of the cell, while no corrosion was observed at the cathode inlet and center and at all anode-side positions. The size of cracks in the microporous layer, especially under the rib area, decreased after cell operation. This study validates that it is possible to observe the cause of fuel cell degradation, i.e., carbon corrosion, at a certain position of the MEA under several stages of operation, without cutting the MEA. (C) 2014 Elsevier Ltd. All rights reserved.