A possibility is described of improving the prediction of ternary liquid-liquid equilibrium data from binary ones by using three-parameter equations for calculating activity coefficients. The parameters for completely miscible pairs of liquids were determined from their vapour-liquid equilibrium data and those for partially miscible pairs of liquids from their mutual solubilities and from one limiting activity coefficient. A quantitatively correct prediction of the liquid-liquid equilibrium data was achieved in all cases by means of three different equations (NRTL equation, Redlich-Kister equation and Wilson equation modified by Novák and coworkers) in four hydrocarbon-hydrocarbon-polar solvent systems investigated.