A portable direct methanol fuel cell stack is characterised, together with a novel methanol-feeding concept. The amounts of water and carbon dioxide in the cathode and anode outlets respectively are measured and compared with calculated values based on parameters found in the literature. Due to methanol crossover, the stack temperature already increases rapidly at open circuit voltage, which also indicates substantial losses. The efficiency of the stack is found by two methods to be somewhat less than 25% at 20W. A passive methanol supply to the feed loop is achieved by placing a permeable tube in a concentrated methanol storage tank. Diffusion of methanol through the tube walls into the methanol–water flow assures an increase in concentration. Experimental investigations of such a tube are compared with simulations.