The interfacial tension against hexane of the aqueous solution of the mixture of sodium decyl sulfate (SDeS) and sodium perfluorooctanoate was measured as a function of the total molality and the composition of surfactants at 298.15 K under atmospheric pressure. The miscibility in the adsorbed film and micelle was examined on the basis of the composition data. The film composition was evaluated from the total molality vs bulk composition curve at a given interfacial tension; the two surfactants are miscible in all proportions and the adsorbed him abounds in SDeS. The micelle composition was evaluated from the critical micelle concentration (CMC) vs bulk composition curve; the two surfactants are partially miscible and the two kinds of micelles having different overall compositions coexist in the micellar solution at the bulk composition of the cusp of the curve. This conclusion was thermodynamically confirmed by evaluating the film composition at the CMC from the micelle composition.