This paper compares direct methanol fuel cells (DMFCs) employing two types of Nafion(R) (E.I. DuPont de Nemours and Company) membranes of different equivalent weight (EW). Methanol and water uptakes in 1100 and 1200 EW Nafion membranes were determined by weighing P2O5-dried and methanol solution-equilibrated membranes. Both methanol and water uptakes in the 1200 EW membrane were about 70-74% of those in the 1100 EW membrane. The methanol crossover rate corresponding to that in a DMFC at open circuit was measured using a voltammetric method in the DMFC configuration and under the same cell operating conditions. After accounting for the thickness difference between the membrane samples, the methanol crossover rate through a 1200 EW membrane was 52% of that through an 1100 EW membrane. To resolve the cathode and anode performances in an operating DMFC, a dynamic hydrogen electrode was used as a reference electrode. Results show that in an operating DMFC the cathode can be easily flooded. as shown in a DMFC using 1100 EW membrane. An increase in methanol crossover rare decreases the DMFC cathode potential at open circuit. At a high cell current density, the DMFC cathode potential can approach that of a H-2/air cull.