The steady-state kinetics of horseradish peroxidase-catalyzed dihydroquercetin oxidation was studied. Dihydroquercetin was shown to be a slowly oxidized substrate of horseradish peroxidase. Two dihydroquercetin isoforms (cis and trans forms) that were selectively involved in peroxidase-induced oxidation were found in water-alcohol and buffer solutions. The k(cat) and K(m) were determined in the pH range of 4.5-8.0.