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6-Substituted 7,8-dihydropterins (¼2-amino-7,8-dihydropteridin-4(1H)-ones) are heterocyclic compounds that occur in a wide range of living systems and participate in relevant biological functions. In airequilibrated aqueous solutions, these compounds react with dissolved O2 (autooxidation). The rates of these reactions as well as the products formed strongly depend on the chemical structure of the substituents. 7,8-Dihydro-6-methylpterin and 7,8-dihydro-6,7-dimethylpterin that bear electron-donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety to yield the corresponding oxidized derivatives (6-methylpterin and 6,7-dimethylpterin, resp.). The oxidations of 7,8-dihydrobiopterin, 7,8-dihydroneopterin, and 7,8-dihydrofolic acid are slower, and they yield 7,8-dihydroxanthopterin as the main product. 7,8-Dihydroxanthopterin, 6-formyl-7,8-dihydropterin, and sepiapterin are rather stable, and their consumption in air-equilibrated solutions is negligible for several days. The pseudo-first-order rate constants of the reactions between these compounds and O2 at 258 and 408 are reported. The biological implications of the results obtained are also discussed.¼2-amino-7,8-dihydropteridin-4(1H)-ones) are heterocyclic compounds that occur in a wide range of living systems and participate in relevant biological functions. In airequilibrated aqueous solutions, these compounds react with dissolved O2 (autooxidation). The rates of these reactions as well as the products formed strongly depend on the chemical structure of the substituents. 7,8-Dihydro-6-methylpterin and 7,8-dihydro-6,7-dimethylpterin that bear electron-donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety to yield the corresponding oxidized derivatives (6-methylpterin and 6,7-dimethylpterin, resp.). The oxidations of 7,8-dihydrobiopterin, 7,8-dihydroneopterin, and 7,8-dihydrofolic acid are slower, and they yield 7,8-dihydroxanthopterin as the main product. 7,8-Dihydroxanthopterin, 6-formyl-7,8-dihydropterin, and sepiapterin are rather stable, and their consumption in air-equilibrated solutions is negligible for several days. The pseudo-first-order rate constants of the reactions between these compounds and O2 at 258 and 408 are reported. The biological implications of the results obtained are also discussed.2 (autooxidation). The rates of these reactions as well as the products formed strongly depend on the chemical structure of the substituents. 7,8-Dihydro-6-methylpterin and 7,8-dihydro-6,7-dimethylpterin that bear electron-donor groups as substituents are the most reactive derivatives and undergo oxidation of the pterin moiety to yield the corresponding oxidized derivatives (6-methylpterin and 6,7-dimethylpterin, resp.). The oxidations of 7,8-dihydrobiopterin, 7,8-dihydroneopterin, and 7,8-dihydrofolic acid are slower, and they yield 7,8-dihydroxanthopterin as the main product. 7,8-Dihydroxanthopterin, 6-formyl-7,8-dihydropterin, and sepiapterin are rather stable, and their consumption in air-equilibrated solutions is negligible for several days. The pseudo-first-order rate constants of the reactions between these compounds and O2 at 258 and 408 are reported. The biological implications of the results obtained are also discussed.2 at 258 and 408 are reported. The biological implications of the results obtained are also discussed.8 and 408 are reported. The biological implications of the results obtained are also discussed.