The Redlich-Kwong equation of State was used to calculate liquid and vapour phase fuga- cities of Acetic acid (l)-n. Heptane (2) and Acetic acid (l)-Benzene (2) Systems for predicting isothermal vapour-Iiquid equilibria. The binary interaction coefficients were obtained by a mi- nimization technique with the help of a Computer. The agreement, between the predicted and experimental equilibrium data allows the calculation of isothermal vapour-liquid data for Sys¬tems containing both polar and non-polar compounds.