Obtaining accurate values for vapor-liquid equilibrium (VLE) is very important in the design of separation equipment and some industrial applications in chemical engineering. So in order to obtain accurate properties of VLE attention has been turned to calculate them from equations of state (EOS), noting that those equations of state are strongly nonlinear as well as inaccurate results may by obtained when they are applied to nonideal and polar systems. To overcome this problem efforts were directed to modify or improve EOS and their mixing and combining rules. In this book, isobaric vapor- liquid equilibrium for ternary systems has been measured at 101.325 KPa. The measurements were made in recirculating equilibrium still with circulation of both the vapor and liquid phases. . In this book the calculation of VLE K–values is done by using the modified Soave Redlich and Kwong (SRK), modified Peng and Robinson (PR) equations of state and the proposed method that uses the new, three-parameter, temperature dependency form alfa function for the attractive term A of the Peng-Robinson equation of state.