The interactions of an aqueous phenazinium dye, viz., neutral red with nonionic Tween surfactants in the presence of some nonionic polymers, viz., PVA, PEG-200, PEG-400 and PEG-600 have been investigated. The equilibrium constants of the interaction of the dye with the micelles in the absence and in the presence of the polymers have been determined by a partition equilibrium (PE) method based on UV-VIS spectroscopy. The distributions of the two forms of the dye between the bulk water and the micelles have been computed from the equilibrium constants by an iterative method using a FORTRAN program. The thermodynamic parameters of the binding of the dye with the micelles and the polymers have been evaluated, which indicate the dye-surfactant associations to be entropy driven. The apparent pK a s of the dye have been predicted from the concentrations of the acid and base forms of the free and bound dye as well as determined directly by spectrophotometric method. The predicted pK a s agreed well with the experimental results in the micellar systems as well as in the aqueous surfactant-polymer systems indicating predictability of the acid-base behavior of the dye by the present method.