For fractionating dextran using the antisolvent ethanol, the phase equilibria of polydisperse dextrans in the ternary system water-ethanol-dextran should be known, especially the influence of the molecular mass of the dextran on the liquid-liquid phase equilibrium. By applying the lattice solution theory of Flory and Huggins with molecular mass dependent interaction parameters, the molecular mass distributions of a dextran in both coexisting phases can be predicted from investigations of narrowly distributed dextrans in the same system. Numerical calculations are in good agreement with the molecular mass distributions of dextran obtained experimentally, using size exclusion chromatography. On this basis, the influence of the concentrations of ethanol and dextran of the total system on the fractionation of dextrans with any molecular mass distributions can be calculated numerically.