The Kontogeorgis' approach (CPA; Ind. Eng. Chem. Res. 1996, 35, 4310) was employed for the extension of the Soave -Redlich - Kwong (SRK) equation to associating systems. The two-state association model (TSAM) of Cerdeirina et al. Q. Chem. Phys. 2004, 120, 6648) was modified and employed for describing the association contribution. The modification of TSAM consisted of developing an expression for the association contribution for the Helmholtz free energy as an explicit function of temperature and density, on the basis of the two-state approach. Similar to the CPA equation, the resulting cubic two-state (CTS) equation of state (EoS) has five parameters, three of them related to the nonspecific (physical) contribution and the other two related to the association. The CTS EoS is polynomial in volume and preserves the shape of a cubic equation in the region of positive volumes greater than the covolume, having one or three roots in this zone. For pure substances the equation is quartic, and it has analytical solution. The equation was fit to experimental vapor pressure and liquid density data of water, alcohols, and phenols, with very low deviations. The model was also employed to predict second virial coefficients of pure compounds, and phase equilibria of alcohol-alkane mixtures, using conventional mixing rules and one binary parameter, with very good agreement with experimental data.