By the application of thermodynamic fundamentals of phase and electrochemical reaction equilibria, a mathematical equation has been developed to predict the reversible open-circuit voltage of a direct methanol fuel cell (DMFC), in the absence of electrode poisoning and methanol crossover. The equation accounts for the effect of the nonideal behavior of the fluid phases on the reversible open-circuit voltage. Sample computed results are presented. The species (CH3OH, H2O, H+) flux equations to compute molar fluxes through the polymer electrolyte membrane of a DMFC are also presented.