Reduced PQQ (PQQH 2) is prepared by the reaction of PQQ with thiophenol, 1-benzyl-1, 4- dihydronicotinamide (BNAH), sodium dithionite, or sodium borohydride. Oxidation of PQQH 2 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 PQQH 2 and H 2 O 2 produced by the reaction of PQQH 2 and O 2. As in fact, ...