Polymer-surfactant complex formation is described by small systems thermodynamics at three structural levels: macroscopic system, small-system, subsystem. Such complexes are treated as semi-open small systems of surfactants and segments of a polymer molecule. The internal structure of the small system is characterized as a fluctuation product of a collection of open subsystems of surfactant aggregates and polymer segments. The theory unifies and shows the equivalence of different (adsorption, pseudophase separation, chemical equilibrium) concepts of descriptions. It explains that the polymer-surfactant interaction does not depend on the polymer molecular weight, and that the mean aggregation number of surfactants in the complex increases with the bound amount. The isotherm equation of the surfactant binding on the polymer together with its thermodynamic analysis is given.