For practical fuel cell applications it is vital to know how the fuel cell operates in varying ambient conditions, especially when passive control methods are used. In this contribution, the effect of subzero temperatures with constant current density and cold-start behavior of planar free-breathing PEMFC were studied in a temperature chamber. The temperature levels used in constant current measurements varied between 0 and -27.5 degrees C. The cell maintained stable operation without irreversible performance losses at higher current densities as the heat generation was high enough to prevent the product water from freezing inside the cell. However, ice formation on the outer part of the cathode side of the cell was observed. At low temperature and current density level, the freezing of product water inside the cell led into irreversible performance loss. The cold-start measurements showed that the cell is capable of starting operation at -5 degrees C without irreversible performance losses when the cell is initially dry. The cell was capable of starting operation also at -10 degrees C when the starting procedure was slow enough, but a slight irreversible performance loss was encountered. (c) 2005 Elsevier B.V. All rights reserved.