ABSTRACT: Spectral changes produced in the oxidation of eriodictyol by peach polyphenol oxidase were followed over time. A product with λmax= 390 nm was seen to appear before another with λmax= 475. The product absorbing at 390 nm must correspond to the o-quinone derived from eriodictyol. The compound absorbing at 475 nm must be derived from this eriodictyol-o-quinone. Progress curves at this wavelength revealed a lag, the length of which varied with enzyme and substrate concentrations. This lag must have been caused by chemical reactions taking place after the enzymatic reaction. When eriodictyol oxidation was studied in the presence of 3-methyl-2-benzothiazolinone hydrazone hydrochloride (MBTH), a potent nucleophilic reagent that reacts with the eriodictyol-o-quinone to form a dark pink product absorbing at 508 nm, the lag disappeared. When the kinetic parameter was evaluated in the presence of MBTH (Km= 0.6 mM), the results was similar to those obtained without MBTH. Eriodictyol oxidation was inhibited by tropolone, which behaved as a classic competitive inhibitor (KI= 15 μM). The inhibition results reported show that eriodictyol oxidation was strictly dependent on the presence of polyphenol oxidase. In addition, other oxidase activities, such as laccase and H2O2 independent phenol oxidase, were not detected in the enzyme extract.