Ice/frost formation in a proton exchange membrane fuel cell (PEMFC) operating under subzero temperatures can lead to its shutdown during start up. Isothermal potentiostatic and galvanostatic tests were performed on 220cm(2) single cells under a wide range of operating conditions in order to investigate the "cold start" behaviour. Different parameters have been investigated: the initial water contained in the membrane, the operating voltage, the cell temperature and current. An optimal wetting level of the fuel cell (FC) core for which cumulated heat generated by the electrochemical reaction is maximal, has been observed. Water management analysis from the membrane coupled with cell resistance measurement allowed to formulate a phenomenological interpretation of the overall FC performance evolution. FC starving is not only due to ice formation in the cathode layer pores, thus hindering oxygen transport. It is also due to ice formation in active reaction sites increasing the electrical resistance of the cell. Both factors dramatically reduce FC performance under load. The relative balance of each effect has been assessed. After each shutdown and start up at subfreezing temperature, a polarization curve at rated conditions was carried out to quantify the FC performance degradation. Performances reduce less than 1% per cold start at rated current density. (c) 2008 Elsevier B.V. All rights reserved.