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.