Reactions of the complexion [Mo2O4(C2O4)2(H2O)2]2– with potential ligands have been investigated. According to continuous variation studies the reacting ratios (ligand : molybdenum) were : with ethylenediaminetetra-acetate and 1,10-phenanthroline, 1 : 2; with pyridine-2-carboxylic acid, 1 : 1; with 8-hydroxyquinoline-5-sulphonic acid, 1 : 1 and 2 : 1. The following compounds have been isolated: (pyH)2[Mo2O4(C2O4)2(H2O)2], (pyH)2[Mo2O4(C2O4)2(py)2], Na2[Mo2O3(C2O4)2(oxinate)2],5H2O, [Mo2O3(oxinate)4],H2O, Na2[Mo2O3(C2O4)2(pic)2],3H2O, and [Mo2O3(C2O4)2(phen)2],2H2O. The ease of replacement of ligands was H2O > bridging oxygen > C2O42– terminal oxygen, and of substitution, 8-hydroxyquinoline > 8-hydroxyquinoline-5-sulphonic acid EDTA picolinic acid > phenanthroline > pyridine. The magnetic properties, infrared and electronic spectra of the complexes, and their probable structures are discussed and also internal ligand transitions in the oxinate complexes. The relevance of the results to molybdenum–substrate interaction in the enzyme xanthine oxidase is also discussed.