The E-PPR78 model is a predictive version of the widely used Peng-Robinson equation of state in which the binary interaction parameters are estimated by a group-contribution method. With the 21 groups available before the writing of this paper, such a model could be used to predict fluid phase equilibria of systems containing hydrocarbons, permanent gases (CO2, N-2, H2S, H-2), mercaptans, alkenes and water. In other words, all the components encountered in petroleum fluids could be modeled. It is however acknowledged that natural gases often contain small amounts of carbon monoxide, helium or argon. In order to model the phase behavior and estimate energetic properties (e.g. enthalpy changes on mixing) of such gases, the applicability range of the E-PPR78 model is extended through the addition of three new groups: CO, He and Ar.