The acidity constants of 14 recently synthesized derivatives of 9,10-anthraquinone in methanol + water at 25 degrees C and an ionic strength of 0.1 M have been determined spectrophotometrically. A linear inverse relationship between the pK(a) of all acids and the mole fraction of methanol in the solvent mixtures is observed. The influence of substituents in the molecular structure on the ionization constants is discussed.