The composition and mutual interaction of the components in anionic-cationic adsorption films have been determined by using a regular solution model for the mixed monolayers. Equimolar mixtures of surfactants that differ markedly in their surface activity (decylpyridinium chloride and sodium butylsulfonate) have been investigated. A marked asymmetry of the composition of mixed adsorption films, as web as mutual attraction of the adsorbed components (negative values of surface interaction parameter beta) have been found in these systems. The effect of the ionic strength and kind of the inorganic electrolyte on the surface interactions is similar, as in the case of mixtures of decylpyridinium chloride and sodium alkylsulfonates with hydrocarbon chains longer than the butyl ones that were previously investigated.