An equation is derived for predicting the interfacial tension of nonideal organic mixtures over water. The equation combines the Gibbs and the nonideal Langmuir adsorption isotherms to describe interfacial tension in terms of organic bulk phase properties. Required inputs are pure component interfacial tension data and the organic bulk phase activity coefficients. The equation successfully predicts the experimental literature data for immiscible and partially miscible ternary systems, including aromatics, aliphatics, alcohols, and ketones. In addition, the equation is a useful tool for predicting the temperature dependence of interfacial tension.