Ice distribution in the catalyst layer and gas distribution layer (GDL) of proton exchange membrane (PEM) fuel cells under isothermal constant voltage (ICV) operation at a subzero temperature was determined using a field emission scanning electron microscope with a cryogenic stage and sample preparation unit (CRYO-FESEM). The analysis method was designed to ensure that the entire experiment, from sample preparation to CRYO-FESEIM characterization, are carried out under subzero (degrees C) conditions so that the water is always kept in a frozen state without thawing. Under a moderately wet shutdown, the porosity of the cathode catalyst layer decreased from an initial dry porosity of 65% to 15.9% for the frozen-only sample and to 8.2% for the sample which was operated at subzero temperatures (ICV). After the completion of the ICV, the catalyst surface was completely covered with ice and the gas was not able to reach the active sites and the reaction ceased. Two distinct regions with different porosities in the catalyst layer were observed at the half ICV state, which indicates that ice in the catalyst layer melted at the beginning of ICV operation. (c) 2008 Elsevier Ltd. All rights reserved.