This work provides the first extensive study of the redox reactivity of the pyranopterin system that is a component of the catalytic site of all molybdenum and tungsten enzymes possessing 2 molybdopterin. The pyranopterin system possesses certain characteristics typical of tetrahydropterins, such as a reduced pyrazine ring, however it behaves as a dihydropterin in redox reactions with oxidants. Titrations using ferricyanide and DCIP (dichlorophenylindophenol) prove a 2 e -/ 2 H + stoichiometry for pyranopterin oxidations.Oxidations of pyranopterin by Fe(CN) 6 3-or DCIP are slower than tetrahydropterin oxidation under a variety of conditions but are considerably faster than observed for oxidations of dihydropterin. The rate of pyranopterin oxidation by DCIP was studied in a variety of media. In aqueous buffered solution, the pyranopterin oxidation rate has minimal pH dependence whereas the rate of tetrahydropterin oxidation decreases 100-fold over the pH range 7.4 -8.5. Although pyranopterin reacts as a dihydropterin with oxidants, it resists further reduction to a tetrahydropterin. No reduction was achieved by catalytic hydrogenation, even after several days.The reducing ability of the commonly used biological reductants dithionite and methyl viologen radical cation was investigated but experiments show no evidence of pyranopterin reduction by any of these reducing agents. This study illustrates the dual personalities of pyranopterin and underscores the unique place that the pyranopterin system holds in the spectrum of pterin redox reactions. The work presented here has important implications for understanding the biosynthesis and reaction chemistry of the pyranopterin cofactor in molybdenum and tungsten enzymes.