Complexes MoOB2(B = diethyldithiocarbamate, diethyldithiophosphate, or diphenyldithiophosphate) have been prepared by reducing oxomolybdenum(V) complexes, Mo2O3B4, with thiophenol, sodium dithionite, and zinc dust. The complexes are diamagnetic and monomeric. Their n.m.r., i.r., and electronic spectra are reported. The physical measurements indicate square-pyramidal structures. Reduction of the oxinate complex, Mo2O3(C9H6ON)4, gave some MoO(C9H6ON)2 but the acetylacetonate complex, Mo2O3(C3H7O2)4, could not be reduced. It appears that oxomolybdenum(IV) is preferentially stabilised by sulphur-donors.