The complex oxomolybdenum(VI) oxalate having the formula MoO 2C 2O 4 · 2H 2O has been prepared and characterized by chemical analysis and infrared spectra and its thermal decomposition has been studied by TG and DSC. The molybdenum oxalate prepared decomposes exothermally between 200 and 300°C in one step. The product of this decomposition is molybdenum oxide, (blue oxide); this compound has been characterized by X-ray powder diffraction and the observed d values are identical with data reported for MoO 3.