A new procedure for extending the applicability of cubic EOS to critical conditions is described. This method requires to determine two sets of optimum binary interaction parameters for the non-critical and critical parts of PTx experimental data of binary systems. The effective binary interaction parameters are calculated by interpolation between these two sets of binary interaction parameters. The interpolation factor is a function of a distance from critical conditions, which is evaluated from an a priori calculation of the molar volumes of coexistent liquid and vapour. The algorithm is shown to improve the prediction of the Soave-Redlich-Kwon equation of state in the ternary CO2-CH4-N-2 system. Application conditions of this approach to other systems are discussed.