electrochemically active Pt area during and post-subzero startup of a polymer electrolyte fuel cell (PEFC). It was found that the Pt area decreases after each cold start and the Pt area loss increases with the product water generated during cold start. We hypothesize that the Pt area loss is caused by ice precipitated between Pt particles and ionomers during cold start. After startup from a subzero temperature and warmup to 25 degrees C with all ice in the CL melted, the cell remains subject to Pt area loss, but this loss at 25 degrees C is substantially reduced. We also find that subsequent cell operation at 70 degrees C and 1 A/cm(2) for 2 h is very effective for recovering the active Pt area and cell performance. Both permanent loss in the active Pt area and cell performance degradation due to structural alteration of the cathode CL by the presence of ice increase gradually with the cold-start cycle number and become more severe for cold start from -30 degrees C than from -10 and -20 degrees C. In addition, startup from a subzero temperature appears to have no influence on the electrochemically active Pt area of the anode CL. It is suggested that the ice amount present in the CL holds a key to determine the temporary Pt area loss due to ice formation as well as permanent performance degradation resulting from cold-start cycling. (c) 2007 The Electrochemical Society.