The composition and mutual interaction of the components in mixed adsorption films have been determined on the basis of the results of surface tension measurements obtained for solutions of single anionic and cationic surfactants with hydrocarbon chains of the same length (sodium decylsulfonate and decylpyridinium chloride) and their mixtures. An equation analogous to one derived on the basis of a regular solution model, however without the controversial assumption that the excess entropy of mixing is zero, has been used in calculations. Investigations have been made for mixtures of different compositions and for different concentrations of added inorganic electrolyte (NaCl).