Pinhole formation in the membrane is a polymer electrolyte membrane fuel cell (PEMFC) failure mode typically due to mechanical causes. This study reports the discovery of thermal pinhole formation caused by a gas leak. The study was prompted by an unexpected cell failure of a real PEMFC stack during its initial conditioning. A little gas leak had been detected in the stack, and the gas leak was suspected to generate pinholes. However, in spite of the large gas leak, pinholes did not appear in the normal cells with a GDL-covered catalyst layer. Additional cells with a GDL-uncovered catalyst layer simulating an uneven electrode edge of the failed stack were then tested under the gas leak conditions. Pinholes were demonstrated to appear at the GDL-uncovered catalyst layer position near the gas leak path during the initial conditioning and the phenomena were reproducible. A gas-leak-induced thermal pinhole formation is suggested in which the leaked gas hits the GDL-uncovered low-heat-dissipation catalyst, a hot spot occurs by catalytic combustion, generating the pinhole. (C) 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.