The open-circuit voltage (OCV) of a direct methanol fuel cell (DMFC) was observed to undergo an overshoot before it stabilized during the startup period of the oxygen supply. This transient OCV overshoot behavior was due to the hydrogen evolution and subsequent oxidation reactions at the anode catalyst layer. Furthermore, within the OCV overshoot period the DMFC anode behaved like a hydrogen electrode due to the existence of hydrogen at the catalyst layer, resulting in a transient reference hydrogen electrode, which allows quantifying the respective anode and cathode potentials of the DMFC. (c) 2005 The Electrochemical Society.