Protonation constants of a number of mono-substituted anilines were determined potentiometrically in 0, 20, 30, 40, 50, and 60% (v/v) dioxane - water mixtures at (25.00 +/- 0.0.02). degreesC at an ionic strength of 0.1 mol-dm(-3) in sodium perchlorate. The data from the potentiometric titrations were evaluated using the BEST computer program. The trends in the values of the protonation constants of anilines were explained in terms of the nature of substituent and the solute - solvent and solvent - solvent interactions. Furthermore, the effects of the substituents on the basicity of aniline, the additivies of these effects, and the applicability of the Hammett equation to the behavior of these substituents are discussed.