The thermal decomposition of [Co(NH3)sCO3]NO3, [Co(NH3)5NO2](NO3)2 and [Co(NH3)sH20](NO3)3 complexes was studied by simultaneous TG, DTG, DTA (Derivatograph-C), termogastitrimetry, continuous selective water detection, TG-MS, and XRD methods.Due to the nearly explosive reaction of these complexes the samples were heated in a labyrinth-type crucible after a 10-fold dilution with alumina.Through the quantitative determination of NH3 and H20 it was possible to reveal the subtleness of decomposition of these complexes.Based on the qualitative and quantitative results obtained by the different thermoanalytical techniques used the equations of decomposition are given for both stages of the reactions.Systematic studies on the thermal decomposition of crystalline pentamminecobalt(III) complexes dates back to a relatively early stage of development of thermoanalytical methods [1][2][3][4]. It was observed that the peak temperature of the exothermic redox reaction of [Co(NH3)5X] 2+ complexes (where X = r, Br-, CI-, F", and NO~-) could be correlated to the ligand field strength of the hetero ligand, X. The remarkable feature of these compounds is that they exhibit a similar trend in their redox reaction induced with electron transfer quenching of the photoexcited Ru(bpy)~ + complex (bpy = 2,2'-bipyridine) [5][6][7][8][9]. The new aspects of chemistry of these compounds directed our attention to the reinvestigation of their thermal reactions using highly sophisticated thermoanalytical techniques.