Direct methanol fuel cells have been characterised under ambient conditions. By operating the cathode with hydrogen as well as air, the half-cell and full-cell overvoltages have been found. Both the anode and the cathode kinetics improve with increasing temperature, but the cathode suffers from additional losses due to methanol crossover and water flooding. The methanol concentration, however, mainly influences the cathode. Impedance spectroscopy measurements on single cells and stacks confirm these results, and indicate that water removal from the cathode is important for stable operation of direct methanol fuel cells. Carbon dioxide (CO2) in the cathode outlet was measured by mass spectroscopy (MS). The amount of carbon dioxide crossover is noticeable compared to the methanol crossover, but decreases with temperature and methanol concentration. Almost 100% of the crossover methanol is converted to carbon dioxide at the cathode. (c) 2006 Elsevier B.V. All rights reserved.