Direct methanol fuel cells (DMFCs) are attractive for several applications, however, at present, methanol crossover from the anode to the cathode appears to be a major limitation. For this reason, as one of the alternatives, membrane technology tries to obtain new methanol-impermeable polymer electrolytes. In this sense, it would be interesting to develop easy methods to check the new materials in relation to the methanol transport. In this work, a simple model is presented which permits to estimate easily the methanol diffusion coefficient of the membrane of a DMFC from open circuit voltage (OCV) measurements. The model has been checked in a DMFC using Nafion membranes as electrolyte.