The examination of the dissociation constants of sulfonamides is essential in drug-design studies and in explaining the biopharmaceutical properties of substances. The dissociation constants of ten common human and veterinary antibiotics, namely, sulfadiazine, sulfamerazine, sulfamethoxazole, sulfathiazole, sulfamonomethoxine, sulfamethoxypyridazine, sulfadimethoxine, sulfafurazole, sulfadoxine, and sulfaquinoxaline, in water and in 15 %, 23 %, and 30 % (v/v) acetonitrile-water mixtures were determined by a UV/pH titration method and correlated with the Kamlet and Taft solvatochromic parameters, pi*, alpha, and beta. Kamlet and Taft's general equation was reduced to two terms by combined factor analysis and target factor analysis in these mixtures: the independent term and polarity/polafizability pi*, which is a solvatochromic parameter. Further, the quasi-lattice quasi-chemical (QLQC) theory of preferential solvation has been applied to quantify the preferential solvation by water of electrolytes in acetonitrile-water mixtures.