Passive direct methanol fuel cells (DMFC) are under development for use in portable power applications due to their enhanced energy density in comparison with active DMFCs. This study has been carried out to understand the unique properties of passive DMFCs, focusing on the internal temperature and the open circuit voltage (OCV), which change as a consequence of the methanol crossover phenomenon. The changes found in the passive DMFCs were very different from active ones because of different reactants supplying conditions. Methanol concentration in the built-in reservoir attached to the anode changed with time on stream and the OCV and the temperature changed correspondingly. Various experiments were conducted to show the unique properties of passive DMFCs at controlled conditions and configurations of the cell. (c) 2004 Elsevier B.V. All rights reserved.