X-ray absorption spectroscopy at the molybdenum K-edge has been used to probe the molybdenum coordination of Rhodobacter sphaeroides dimethyl sulfoxide reductase. The molybdenum site of the oxidized protein possesses a novel Mo(VI) mono-ore site (Mo=O at 1.68 Angstrom) with additional coordination by approximately four thiolate ligands at 2.44 Angstrom and probably one oxygen or nitrogen at 1.92 Angstrom. The reduced Mo(IV) form of the enzyme is a des-oxomolybdenum with 3-4 thiolates at 2.33 Angstrom and two different Mo-O/N ligands at 2.16 Angstrom and 1.92 Angstrom. Similarly, the stable Mo(V) glycerol-inhibited species is found to be a des-oxomolybdenum with approximately four thiolate ligands at 2.40 Angstrom and (probably) two similarly coordinated oxygen or nitrogen ligands at 1.96 Angstrom.