Cold start characteristics of a polymer electrolyte membrane fuel cell are investigated experimentally, and microscopic observations are conducted to clarify the freezing mechanism in the cell. The results show that the freezing mechanism can be classified into two types: freezing in the cathode catalyst layer at very low temperature like -20 degrees C. and freezing due to supercooled water at the interface between the catalyst layer and the gas diffusion layer near 0 degrees C like -10 degrees C. The amount of water produced during the cold start is related to the initial wetness condition of the polymer electrolyte membrane, because water absorption by the membrane due to back diffusion plays an important role to prevent the water from freezing. It is also shown that after the shutdown of the cold start the cell performance of a subsequent operation at 30 C is temporarily deteriorated after the freezing at -10 degrees C, but not after the freezing at -20 degrees C. The ice formed at the interface between the catalyst layer and the gas diffusion layer is estimated to cause the temporary deterioration, and the function of a micro porous layer coating the gas diffusion layer for the ice formation is also discussed. (C) 2012 Elsevier B.V. All rights reserved.