A study of a vapor feed DMFC using PBI loaded with phosphoric acid is reported. The anode catalyst was a Pt-Ru alloy and the cathode Pt, both supported on carbon black. Performance of the fuel cell with low methanol concentrations is reported and in situ measurements of anode and cathode potentials were used to diagnose the fuel cell performance. The influence of temperature, methanol feed concentration, and oxygen pressure are reported. The fuel cell performance was quite low with peak power densities of 12 to 16 mW cm−2 obtained at a temperature of 175 °C, although open circuit potentials of up to 800 mV were achieved. The poor performance was attributed to significant anode polarization due to the presence of phosphate in the catalyst layer and to the influence of methanol crossover on the cathode performance. The performance of the DMFC was found to fall steadily with time over seven days of operation which was associated with an increased cell resistance as measured by ac impedance.