The acid–base equilibria of three sulphonephthalein indictors, viz. bromocresol green, bromothymol blue and phenol red, in 1,4-dioxane–water mixtures and aqueous solutions of both non-ionic and cationic micelles have been studied. The difference between the acid–base equilibrium constant of a sulphonephthalein indicator in pure water, pK^w_a, and the apparent acid–base equilibrium constant of a sulphonephthalein located entirely within the interfacial microenvironment of a non-ionic micelle has been explained quantitatively in terms of the different intrinsic solvent properties of the two systems. A number of factors are considered to be primarily responsible for the difference between the apparent acid–base equilibrium constant of a sulphonephthalein residing with in the interfacial micro-environment of a cationic alkyl quaternary ammonium micelle and its pK^w_a value. These factors are the electrostatic micellar surface potential, the low effective interfacial dielectric constant and either an interfacial ‘salt-effect’ or the formation of an ion pair between the negatively charged sulphonate group of a sulphonephthalein and a cationic quaternary ammonium surfactant headgroup.