Mixtures of ionic and nonionic surfactants have a wide range of industrial and household applications. The properties of the monolayer formed by such systems at the air-water interface depend on the interactions between the surfactants. The presence of salt influences the interactions between the surfactants in the monolayer as well as in the micelle. In this work, these interactions in the presence and absence of a 1:1 salt were studied using nonideal solution theory. An anionic surfactant, a cationic surfactant, and a nonionic surfactant were used. The stability of foams was significantly affected by the interactions between the surfactants. To study this effect, the coalescence of air bubbles in the mixed-surfactant systems was studied and compared with that in the corresponding single-surfactant systems. The effect of salt on the coalescence time was also studied. The results were analyzed using the film drainage and stochastic theories of coalescence. Seven film drainage models were employed 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.