Heavy oil has recently become an important resource, as conventional oil reservoirs have limited production and oil price has risen. More than 6 trillion barrels of oil in place have been attributed to the world’s heaviest hydrocarbons. Therefore, heavy oil reserves account for more than three times the amount of combined world reserves of conventional oil and gas. Heavy oils cover a large range of API gravities, from 22 to less than 10. This wide range of values means that heavy oils vary greatly in their physical properties. Thus, extensive research is required before the properties of heavy oil can be properly understood. Molecular thermodynamics is an engineering discipline rooted in molecular physics and physical chemistry and aimed at developing practical models that predict the properties of matter. The goal of this work is to develop a molecular thermodynamic model for predicting fluid densities, phase equilibria, and energy functions, such as enthalpy. Chemical engineers need these properties to develop and design new processes and materials, and to make these processes energy efficient and environmentally benign. The approach is to hypothesize a mathematical approximatical approximation that captures interactions among real molecules on the basis of a molecular theory called the statistical associating fluid theory (SAFT). In this approximation, each molecule is composed of spherical segments that interact according to a square-well potential. In this study, a modified simplified statistical associating fluid theory (SSAFT) equation of state has been applied to predict the phase equilibrium behavior of associating and non-associating compounds, and their binary mixtures. The new equation of state has the same number of adjustable parameters as the original SAFT equation. In order to study multicomponent systems, the equation is first applied to pure fluids. Three molecular parameters are required to obtain the thermodynamic properties of pure substances namely numbers of segments (m), temperature independent segment molar volume in a closed-packed arrangement (v?), and depth of the square well potential (u). Two additional parameters are required to describe the associating molecules, the association energy, and volume. The experimental data of liquid densities and vapor pressure for pure fluids studied in this work were used to obtain the best values for the parameters of the new version of SSAFT. The result obtained from the new version of SSAFT, for liquid densities and vapor pressure of pure associating and non-associating fluids were compared with those obtained from the original SSAFT equation of state. The results showed that the new version of SSAFT can also accurately correlate the experimental data of liquid density and vapor pressure for systems studied. Binary mixtures are also investigated and reasonable results are obtained with no additional binary parameters. Finally, the new SSAFT equation of state was applied for prediction of phase behavior of heavy oil that showed good accuracy for this issue. The results show that the accuracy of the new equation of state is higher than SSAFTEOS,so that therelative errorofthenewequation of statecompared toexperimental values??in thecalculation ofthe vapor pressureandliquiddensityofpure substancesrespectively30 and25 percentimprovementcompared to theequation of stateSSAFT,respectively,1.782and1.583have been reported. Key words : Equation of state, phase behavior, associating fluids, SAFT, binary mixture