Calcium(II)bis(oxalato)calcium(II)dihydrate, Ca[Ca(C2O4)(2)]. 2H(2)O, has been synthesized and characterized by elemental analysis, and reflectance and IR spectral studies. Thermal decomposition studies (TG, DTG, and DTA) in air showed that the final end product was CaO at 865 degrees C through the intermediate formation of a mixture of CaCO3 and CaO at approx. 818 degrees C. At around 424 degrees C a mixture of CaO and Ca(C2O4)(1.2) is formed by partial decomposition of oxalate which is stable up to 670 degrees C. DSC studies up to 700 degrees C in nitrogen at 10 degrees C min(-1) showed both endothermic and exothermic peaks with activation energies of 190 and 458 kJ mol(-1), respectively, and compared with values calculated from TG. Some of the decomposition products were identified by spectral, analytical and X-ray powder diffraction studies and a tentative mechanism for the decomposition is proposed.