Surface tension measurements have been made on aqueous solutions of anionic monoesters of an alternating copolymer of maleic acid and styrene, MAS-n,with n = 0-10, in the presence of alkyltrimethylammonium surfactants. It was found that the surface activity of these mixtures is increased relative to that observed for the pure components. This synergistic effect was ascribed to the formation of a complex, and its dependence of the complex structure was studied. The standard free energy of adsorption per methylene group was found to be -1.4 kJ mol(-1) for complexes formed with MAS. This value decreases for MAS-n, becoming equal to zero for longer linear alkyl chains. However, all the calculated values of the standard free energy were negative and lower than those obtained for the pure surfactant. These results indicate that the main contribution to this change of free energy is the neutralization of the negative charge by a hydrophobic cation.