Abstract A simple modification of the NRTL equation proposed in a previous work is the basis for a new predictive method to calculate VLE of binary liquid systems. The only analytical modification of the NRTL consists in inserting the ratio of the molar volumes of the pure compounds as a multiplying factor of the binary parameters Gij and Gji. Further, the modified NRTL equation is applied in the one parameter form according to the method proposed by Tassios. The proposed procedure entails in some cases the following steps: (i) Energy parameters gii and gjj for the i–i and j–j interaction are calculated from the enthalpy of vaporization of pure compounds. (ii) A modified form of the geometric mean is assumed for the cross-interaction parameters gij and gji, namely g ij =g ji =K ij (g ii g jj ) 0.5 . (iii) Kij is calculated for each class of compounds by simple rules related to well-known properties of pure compounds. (iv) Systems are grouped in the following classes according their non-ideality: • nearly ideal systems, • alcohols/polar compounds, • polar/polar compounds, • alcohols/non-polar compounds, • polar/non-polar compounds, • miscible aqueous systems, • alcohols/non-polar systems with a miscibility gap. The results show that the simple and general proposed method can be used as a rule of the thumb for a large number of binary systems.