The acid-base equilibria of the sulfonephthalein dyes, namely, bromothymol blue, thymol blue, and cresol red, in aqueous nonionic micellar solutions of Triton X-100, Tween-20, Tween-40, Tween-60, and Tween-80 have been investigated. The equilibrium constants of the partition of the dyes between micellar and aqueous pseudophases have been determined spectrophotometrically at fixed pH. The K-ass increased with the surfactants in the order Tween-80 < Tween-60 < Tween-40 < Tween-20 < TX-100. The effect of such associations on the pK(a2) of the dyes at various concentrations of the nonionic surfactant has been studied. The pK(a2) of the dyes as a function of the concentrations of the surfactant was predicted from the equilibrium constants using a method developed to fit nonionic surfactants and was compared with experimental values determined directly. The pK(a2)S of the dyes predicted according to the present method have been found to be in good agreement with the experimental values. (c) 2004 Elsevier Inc. All rights reserved.