The fate of catharanthine and vindoline during the peroxidase-mediated enzymatic synthesis of α-3 ,4 -anhydrovinblastine (AVLB) has been studied. The results showed that catharanthine and vindoline are suitable electron donors for the oxidizing intermediates of horseradish peroxidase (compound I and compound II). The reduction of peroxidase compound II is one of the limiting steps in the coupling reaction. In fact, k 3 (compound II reduction constant) values were 2.53 10 3 m - 1 s - 1 and 3.57 10 3 m - 1 s - 1 for catharanthine and vindoline, respectively. These values are extremely low if we compare them with those found for other substrates of peroxidase-catalyzed oxidations. From these results and the analysis of the reaction products in conditions in which coupling occurred, we propose a mechanism where vindoline-free radicals resulting from reaction of vindoline with the oxidized states of peroxidase are deactivated by reaction with catharanthine. Catharanthine radicals actually set off the coupling reaction.