Abstract: By combining nonideal mixed adsorption theory with Szyszkowski equation, the surface tension equations for nonideal binary surfactant mixture solution are established and the method to find out equation parameters is proposed. Thus it is possible to predict the surface tensions of binary surfactant mixture solutions at different bulk compositions and concentrations based on the logC relationships of individual surfactants and by only one surface tension measurement of their binary mixture solution. Good agreements have been obtained between predicted and experimental results when the equations established are applied to several anionic/cationic binary surfactant mixtures without or with excess added inorganic salt, which shows that the surface tension equations are generally applicable to nonideal binary surfactant mixture solutions.