When transesterifying alcohols with fats and oils to produce biodiesel, the phase behavior and respective distribution of catalyst and alcohol between liquid phases can significantly impact both reaction rates and product workup. To better understand this phase behavior, the distribution of methanol and catalysts, potassium hydroxide and sulfuric acid, between the biodiesel and alcohol phases was experimentally investigated and modeled. Experimental vapor-liquid equilibrium data were modeled by using the Wilson activity coefficient model and two temperature-independent model parameters are used to obtain good agreement between calculated and experimental data. The distribution coefficients of methanol between biodiesel and glycerin phases were accurately predicted by the VLE activity coefficients and Wilson model. (c) 2005 American Institute of Chemical Engineers