The dissociation constants of six common human and veterinary antibiotics, namely, erythromycin, roxithromycin, tilmicosin, oleandomycin, josamycin, and spiramycin in 15 %, 25 %, 40 % and 50 % (v/v) methanol-water solvent mixtures were determined by UV/pH titration and correlated with the Kamlet and Taft solvatochromic parameters, pi (au), 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/polarizability pi (au), which are solvatochromic parameters. The influence of methanol on the dissociation constants was investigated. Further, the quasi-lattice quasi-chemical (QLQC) model of preferential solvation has been applied to quantify the preferential solvation by water of electrolytes in methanol-water mixtures.