The cell performance of direct methanol fuel cells (DMFC) is 0.5 V at 0.5 A cm(-2) under high pressure oxygen operation (3 bar abs.) at 110 degreesC. However, high oxygen pressure operation at high temperatures is only useful in special market niches. Therefore, our work has now focused on air operation of a DMFC under low pressure (up to 1.5 bar abs.). At present, a power density of more than 100 mW cm(-2) can be achieved at 0.5 V on air operation at 110 degreesC. These measurements were carried out in single cells with an electrode area of 3 cm(2) and the air stoichiometry only amounted to 10. The effects of methanol concentration and temperature on the anode performance were studied by pseudo half cell measurements and the results are presented together with their impact on the cell voltage. A cell design with an electrode area of 550 cm(2), which is appropriate for assembling a DMFC stack, was tested. A three-celled stack based on this design revealed nearly the same power densities as in the small experimental cells at low air excess pressure and the voltage-current curves for the three cells were almost identical. At 110 degreesC a power output of 165 W at a stack voltage of 1.5 V can be obtained in the air mode.