In this paper we report the results of molybdenum K-edge X-ray absorption studies performed on the oxidized active site of xanthine oxidase at pH 6 and 10. These results indicate that the active site possesses one terminal oxygen ligand (Mo = O), two thiolate ligands (Mo-S), one terminal sulfido ligand (Mo = S), and one Mo-OH moiety. EXAFS analysis demonstrates that the Mo-OH bond shortens from 1.97 angstrom at pH 6 to 1.75 angstrom at pH 10, which is consistent with the generation of a Mo-O- moiety. This study provides convincing structural evidence that the catalytic oxygen donor at the oxidized active site of xanthine oxidase is Mo-OH rather than the Mo-OH2 ligation previously suggested by X-ray crystallography. These results support a mechanism initiated by base-assisted nucleophilic attack of the substrate by Mo-OH.