The stoichiometric protonation constants (logbeta) of aniline derivatives were determined potentiometrically over a wide range of solvent composition (0 - 0.74 mole fraction of ethanol). To explain the variation of the logbeta values obtained over the whole composition range studied, the quasi-lattice quasi-chemical theory of preferential solvation was applied. The results were discussed in terms of macroscopic properties of the mixed solvent and different microscopic parameters, such as the Kamlet - Taft solvatochromic parameters to identify the solvent characteristics affecting the logbeta values. Kamlet and Taft's general equation was reduced to two terms by using both multiple-linear regression analysis and combined factor analysis and target factor analysis in these mixtures: the independent term and the hydrogen-bond donating ability alpha (HBD), which is a solvatochromic parameter. Hammett's reaction constant for the protonation of anilines has been obtained for all the solvent mixtures and correlates well with alpha (HBD) of the solvent.