A new method has been developed for predicting liquid activity coefficients in ternary mixtures from group contributions. In this method, activity coefficients are obtained by applying the Gibbs-Duhem equation to the excess Gibbs free energy of mixing. Gibbs free energies of the mixture and pure component fluids are calculated from an expansion about a reference fluid in powers of ratios of hard sphere diameters and molecular potentials. When the pure component differs from the reference by one additional structural group, these ratios represent the size contribution and the attraction contribution respectively of this group to the thermodynamic property of the pure fluid. Deviations of the reference hard sphere property to that of the fluid being modeled are corrected for by use of hard sphere equations of state. Results indicate that this method is useful for predieting activity coefficients of ternary mixtures from binary activity coefficient data.