This work experimentally explores the fundamental characteristics of a polymer electrolyte fuel cell (PEFC) during subzero startup, which encompasses gas purge, cool down, startup from a subfreezing temperature, and finally warm up. In addition to the temperature, high-frequency resistance (HFR) and voltage measurements, direct observations of water or ice formation on the catalyst layer (CL) surface have been carried out for the key steps of cold start using carbon paper punched with microholes and a transparent cell fixture. It is found that purge time significantly influences water content of the membrane after purge and subsequently cold-start performance. Gas purge for less than 30 s appears to be insufficient, and that between 90 and 120 s is most useful. After gas purge, however, the cell HFR relaxation occurs for longer than 30 min due to water redistribution in the membrane-electrode assembly (MEA). Cold-start performance following gas purge and cool down strongly depends on the purge time and startup temperature. The cumulative product water measuring the isothermal cold-start performance increases dramatically with the startup temperature. The state of water on the CL surface has been studied during startup from ambient temperatures ranging from -20 to -1 degrees C. It is found that the freezing-point depression of water in the cathode CL is 1.0 +/- 0.5 degrees C and its effect on PEFC cold start under automotive conditions is negligible. (c) 2007 Elsevier Ltd. All rights reserved.