A new, simple, empirical equation for G(E) (excess Gibbs free energy) of electrolyte solutions is proposed in which, contrary to the commonly used Pitzer equation, binary and ternary interaction parameters relate to the interactions of electrolytes in a solution rather than to the interactions of real species in a solution (i.e., anions, cations and nondissociated molecules). Such an approach radically reduces the number of parameters in the new equation for G(E) as compared with the Pitzer equation and consequently significantly simplifies their calculation. The efficiency of the new approach is demonstrated on the example of modelling the vapour-liquid equilibria of the industrially important and widely investigated NH3-CO2-H2O system.