A new group contribution model is established to describe the cell voltage of a direct methanol fuel cell as a function of the current density. The model equation is validated with experimental data over a wide range of methanol concentrations and temperatures. The proposed model focuses on very unfavorable conditions for cell operation, that is, low methanol solution concentrations and relatively low cell temperatures. The proposed group contribution method includes a methanol crossover effect that plays a major role in determining the cell voltage of a direct methanol fuel cell. (C) 2008 Wiley Periodicals, Inc. J Appl Polym Sci 110: 3186-3194, 2008