Major commercial uses of surfactants involve a mixture of these compounds. The properties of the monolayer formed at the water-hydrocarbon interface depend upon the interaction between the surfactant molecules. The presence of salt influences the interactions between the surfactants in the monolayer and in the micelle. In this work, these interactions in presence and in absence of a 1:1 salt were studied and the results were analyzed using the nonideal solution theory. An anionic, a cationic and a nonionic surfactant were used. The stability of emulsions is significantly affected by the interaction between the surfactants. To study this effect, coalescence of cyclohexane drops at a flat water-cyclohexane interface was studied in the mixed surfactant systems, and the results were compared with single surfactant systems. The effect of salt on coalescence time was studied. The results were analyzed using the film drainage and stochastic theories of coalescence. Seven film drainage models were used to predict the coalescence time, and the values predicted by these models were compared with the experimental data. The parameters of the stochastic model were analyzed based on the properties of the systems (C) 2010 Elsevier Ltd. All rights reserved.