Abstract Reduced PQQ (PQQH2) is prepared by the reaction of PQQ with thiophenol, 1-benzyl-1,4-dihydronicotinamide (BNAH), sodium dithionite, or sodium borohydride. Oxidation of PQQH2 to PQQ by molecular oxygen in aqueous solutions is investigated kinetically. The oxidation is accelerated gradually with proceeding of the reaction, which may be attributed to the side reaction of PQQH2 and H2O2 produced by the reaction of PQQH2 and O2. As in fact, the yield of H2O2 is found to be 50% based on PQQH2. Initial rate is first-order in oxygen concentration. The pH-rate profile suggests that an active species in the reaction is PQQH−. Autocatalysis of O2\ewdot and PQQ itself is scarcely detected in this reaction. The mechanism of the oxidation is also discussed.