A direct methanol fuel cell (DMFC) model has been developed and experimentally verified, with which fundamental calculations of the DMFC were carried out. Modelling comprises the mass transport of the gases in the diffusion layers and catalyst layers, mass transport in the membrane, as well as the reaction and the potential distribution in the catalyst layers. The performance of the fuel cell is adversely influenced by methanol permeation from the anode to the cathode. Moreover, the formation of a mixed potential is possible both at the anode and cathode and has a large negative effect on the energetic performance of the fuel cell. The model provides information concerning the impact of methanol permeation through the membrane on energy and mass yield, and on the influence of the operating and structural parameters.