The pK(a) values of nine polyphenolic acid substances in water and acetonitrile-water mixtures using potentiometric, spectrophotometric, and liquid chromatographic (LC) measurements, have been determined. Also, a new method based on the absorbance spectra at the maximum of chromatographic peaks previously obtained has been applied. This method can be applied to data obtained from LC-UV (diode array detection (DAD)) instruments and retains all the advantages of LC and spectrophotometric methods, such as the possibility of working with impure samples. Finally, the advantages of the different methods to the determination of pK(a) values of nine polyphenolic acids have been described in order to verify the applicability of the proposed methods in aqueous and acetonitrile-water media. (C) 2003 Elsevier Science B.V. All rights reserved.