Dimethyl sulfoxide is the substrate to the molybdenum-dependent enzyme dimethyl sulfoxide reductase, which is a member of the large group of molybdenum-containing non-nitrogenase redox enzymes. The active site of these enzymes is thought to possess a so-called molybdopterin, a hydrogenated pterin with an unusual side chain containing a dithiolene group. Up to now the enzyme reactivity was mostly attributed to molybdenum and to the coordination of these sulfur Ligands in the side chain. The pterin moiety was not taken into account as playing an active part essential for the enzyme reaction. We demonstrated for the first time a possible coordination of a hydrogenated pterin to molybdenum with a complex of quinonoid-dihydro-L-biopterin bound to molybdenum in the oxidation state +IV. Now we report the synthesis, crystal structure, and spectroscopic data for trichloro(quinonoid-N(8)H-6,7-dihydropterin)oxomolybdenum(IV), [MoOCl3(H+-q-H(2)Ptr)] (1) (dihydropterin = H(2)Ptr).(1) Crystal data : a = 9.966(3) Angstrom, b = 14.408(4)Angstrom, c = 17.362(5) Angstrom, V = 2493(2) Angstrom(3), Z = 8, orthorhombic, space group Pbca, R(1) = 0.059 and wR(2) = 0.150. 1 is synthesized in a redox reaction between Mo(VI)O2Cl2 Land tetrahydropterin [H(4)Ptr . 2HCl] and contains a cationic quinonoid dihydropterin coordinated via the N(5) and O(4) atoms to the molybdenum atom. The crystal structure of 1 containing the hydrogenated pterin exhibits an unusually short Mo-N(5) bond length of 2.013(3) Angstrom as compared to 2.324(6) Angstrom for the corresponding bond in oxidized pterin. 1 is able to quantitatively reduce the substrate dimethyl sulfoxide to dimethyl sulfide under strict exclusion of oxygen. This reaction can be monitored by C-13-NMR spectroscopy. A simplified in vivo reaction cycle for the enzyme center of DMSO reductase is proposed as a working hypothesis.