Ionization constants of some flavanols (catechin and epicatechin) and flavonols (kaempherol, fisetin, morin, and quercetin) are determined by capillary zone electrophoresis (CZE). This technique allows the determination of pK(a) values until about 12. The pK(a) values obtained are compared with those calculated by the SPARC computational program. This program predicts the microscopic and macroscopic pK(a) values and the order of deprotonation of the different -OH groups. While for catechin and epicatechin the first ionizable OH group occurs in ring 1 and the second ionizable group in ring 2, in flavonols the first deprotonation occurs in ring 2 and the second in ring 1.