Direct methanol fuel cell performance curves were obtained as a function of three parameters, (1) temperature, (2) fuel flow-rate and (3) concentration. Methanol crossover was measured by gas chromatography as a function of these three parameters at 100 mA/cm(2) in the single-pass fuel delivery mode. The data was used to model a continuous loop mode where pure methanol is injected into a loop that circulates through the flow-field and recovers water from the cathode. The modeled loop composition is identical to the fuel stream used in the single-pass experiments (dilute aqueous methanol). The model results, presented in three-dimensional surfaces, elucidate the impact of parameter variations on the energy and power density of the direct methanol fuel cell (DMFC) and the link between those two figures of merit. In addition, a reasonable estimate of the contribution of mass transport effects due to the carbon fabric current collectors is made along with in situ CO stripping experiments on membrane electrode assembly (MEA) anode surfaces. The analysis shows that, at present, serious compromises are required if reasonable energy and power densities are to be simultaneously maintained in DMFCs using Nafion(TM) 117 as an electrolyte.