The thermal decomposition of chromium(III) formate pentahydrate, Cr-3(OH)(2)(HCO2)(7). 5H(2)O, in helium atmosphere and 20% O-2 in helium atmosphere has been successfully studied by means of TG-DTA-MS, i.e., thermogravimetry-differential thermal analysis (TG-DTA) coupled with evolved gas analysis (EGA) using mass spectrometry (MS). The TG-DTA-MS is useful to interpret the complicated successive reactions and to determine the mechanism of the thermal decomposition. The thermal process in Iie atmosphere proceeded by four steps, while in 20% O-2-He atmosphere it proceeded by three steps, The decomposition scheme of Cr-3(OH)(2)(HCO2)(7). 5H(2)O in He atmosphere was proposed by the following decomposition mechanism via formation of three intermediates: Cr-3(OH)(2)(HCO2)(7) . 5H(3)O --> Cr-3(OH)(2)(HCO2)(7) . 2H(2)O + 3H(2)O Cr-3(OH)(2)(HCO2)(7) . 2H(2)O --> Cr-3(OH)(2)(HCO2)(7) + 2H(2)O Cr-3(OH)(2)(HCO2)(7) --> Cr2O3 + Cr(HCO2)(3) + CO + 3CO(2) + 3H(2) 2Cr(HCO2)(3) --> Cr2O3 + 3CO + 3CO(2) + 3H(2). After the decomposition, a slight mass loss was observed with a sharp exothermic crystallization of Cr2O3 due to evolution of H-2. In 20% O-2-He atmosphere, a drastic exothermic mass loss accompanying evolution of CO2, H2O and H-2 was explicable by one step combustion of the dehydrated formate, and a small amount of mass loss observed with a crystallization of Cr2O3 was attributed to evolution of O-2. The decomposition process without an intermediate was indicated by the temperature increase of the specimen produced by combustion heat.