The acid–base behaviour of two azine derivatives, viz. neutral red and acridine, in aqueous non-ionic Brij-35 and n-octyl β-D-glucoside micellar solutions, in aqueous anionic sodium dodecyl sulphate micellar solutions and in 1,4-dioxane–water mixtures have been investigated. The factors primarily responsible for the difference between the pK_avalue of an azine derivative in pure water and its apparent pK_a value when it is located quantitatively within the micelle–aqueous solution interface have been determined. For the non-ionic micellar systems, the difference can be explained solely in terms of the different intrinsic solvent properties of the two solvating media. For the anionic micellar systems three major factors are considered to be responsible for the difference. These factors are the electrostatic micellar surface potential, the interfacial solvent characteristics, and specific molecular interaction between the cationic protonated moieties of the azine derivatives and the anionic surfactant headgroups.