The composition and mutual interaction of the components in anionic-cationic adsorption films have been determined by using to the mixed monolayers a regular solution model. The effect of the inorganic electrolyte concentration and kind on the properties of monolayers adsorbed from solutions containing decylpyridinium chloride and sodium decylsulfonate in concentration ratios varying over a wide range has been investigated. The calculations have been performed both neglecting and taking into consideration the effect of surface interactions on the partial molar areas of surfactants in the mixed monolayer. It has been found that although calculation results are very different, all conclusions concerning the effect of the solution composition are almost the same in both cases.