The potential of the oxygen cathode in a direct methanol fuel cell is strongly influenced by the crossover of methanol through the poly-electrolyte membrane. In the presence of methanol, oxygen is reduced at the cathode already at open circuit and the equivalent amount of methanol is oxidized. This results in the formation of a mixed potential, up to 200 mV negative to the original oxygen potential. In this work, the anode and cathode potentials of a DMFC are monitored in situ, using a dynamic hydrogen electrode (DHE). For the first time, the effect of crossover on the cathode potential as function of time is presented. Methanol and ethanol as fuels are compared. Changing from methanol to hydrogen, the influence of methanol crossover on the cathode potential can also be followed as function of current density. It is already known that, in addition to the consumption of fuel and oxygen with the formation of a mixed potential, a purely chemical reaction takes place at the platinum surface. A quantitative determination of the respective CO2 formation is presented here.