The variation in interfacial tension in acidic oil-alkaline solution systems has been modeled previously. These existing models account for the adsorption of acid, in situ generated surfactant and the added surfactant at the oil-water interface. However, these models do not account for the interactions that exist between these surfactants in the adsorbed layer. An existing model for the formation of non-ideal mixed micelles has been modified and extended to a three component mixture to quantify these interactions. The model is applied to an experimental system consisting of an acidic oil in contact with an alkaline solution containing surfactant. Such systems are encountered in the process of chemical enhanced oil recovery. A good agreement is observed between the experimentally measured interfacial tensions and the values predicted by the present model. The best fit estimates of the monolayer interaction parameters between the acid, surfactant and ionized acid are reported.