Liquid-feed direct methanol fuel cell systems (DMFC) have a number of advantages over hydrogen fuel cells. For a DMFC no additional fuel processing is necessary, therefore, it has a lower volume and a lower weight, the existing infrastructure for fuel supply and distribution can be utilized and the fuel costs are low. However, a number of technical problems have still to be solved. The most important are the cross-over of methanol through the polymer electrolyte membrane, the removal of carbon dioxide from the anode catalyst layer, and the poor anode kinetics. These aspects are analysed by means of a steady state, isothermal cell model which accounts for the essential mass and charge transport processes in the different fuel cell layers. The model is applied to evaluate experimental current-voltage data which were obtained from a small scale cell fuelled with liquid methanol/water solutions.