An interfacial activity model has been proposed to predict the equilibrium interfacial tension of acidic oil/surfactant-enhanced alkaline systems. The model accounts for a mixed interfacial layer and mixed micelle formation by the acid, ionized acid, and added surfactant. The mass action model was adopted for the formation of micelles in conjunction with an empirical relationship for the critical micelle concentration change. The agreement between model predictions and experiments is satisfactory and confirms the mechanisms proposed. For the acidic oil/alkali system, it has been found that the unionized acid adsorbs onto the interface along with the ionized acid, resulting in lower interfacial tension, but fails to produce ultralow values because the ionic strength is not constant as the pH is varied. It has been also found that the acid lowers the interfacial tension of the oil/added surfactant systems by simultaneously adsorbing onto the interface and reduces the critical micelle concentration of the added surfactant.