AbstractThe interaction of co‐polymers of vinylpyrrolidone‐vinylacetate with anionic surfactants, such as lithium dodecyl sulfate (LiDS), lithium perfluorooctane sulfonate (LiFOS) in aqueous solution, has been studied. When the content of vinylacetate in the co‐polymers increases, reduction in the surface tension of co‐polymers alone becomes significant. In mixtures of co‐polymers and surfactants, co‐polymer‐LiFOS complexes are formed at lower surfactant concentration than that of co‐polymer‐LiDS. The micropolarity of the co‐polymers‐surfactant complexes depends on the composition of co‐polymers and is higher for co‐polymer‐LiFOS than that for co‐polymer‐LiDS. Further, the solubilization behavior of α‐(o‐tolylazo‐)‐β‐naphthylamine (Yellow OB) (Tokyo Kasei Co., Ltd., Tokyo, Japan) in the co‐polymer‐surfactant complexes is almost independent of the co‐polymer composition, but different from the surfactants, where a very low solubilized amount of Yellow OB is observed for co‐polymer‐LiFOS.