A new statistical thermodynamic model for liquids has been proposed. A viscosity equation for pure liquids, a viscosity equation for liquid mixtures, and an equation for activity coefficients are obtained from the model and are expressed by the same parameters. The model can be used to correlate viscosities of pure liquids at different temperatures, to simultaneously correlate viscosities and activity coefficients of binary liquid mixtures at different temperatures and compositions, and to predict viscosities and activity coefficients of multicomponent systems. Finally, for binary systems viscosities can be predicted from activity coefficient information and vice versa. The results show that both the correlations and the predictions with the new model are very good