The protolytic equilibria of 13 4-aryl-2,4-dioxobutanoic acids (ADKs) were spectrophotometrically studied in aqueous solutions in the pH range 1-9 at 25±1°C and an ionic strength of 0.1 mol l (NaCl), with the exception of the 4-OH-derivative which was also potentiometrically studied in the pH range 7-10 at 25±1°C and an ionic strength of 0.1 mol l (NaCl). In solution, the compounds simultaneously exist in one diketo and two enolic forms; therefore, the determined acidity constants (pZ 1.87-2.29, pK 6.63-8.13 and pK(4-OH-) 9.52) represent system macro constants. The H-NMR spectrum of the parent compound (4-phenyl-2,4-dioxobutanoic acid) (25°C, pD 5.0) proved the existence of all tautomeric forms. Using the extended Hammett relation, the determined pK values were correlated with literature σ values. The predicted pK values were in fair accordance with the experimentally observed ones. Molecular, monoanionic and dianionic forms of the parent compound were optimized by the semi-empirical molecular orbital PM6 method using the implicit water solvation model (COSMO). The obtained geometries were used to explain the quality of the LFER models.