The thermal decomposition of halogenopentaammine- and cis-dmalogenotetraammineruthenium(III) complexes was studied by the techniques of thermogravimetry(TG), differential thermal analysis(DTA), and evolved gas analysis (EGA). The stoichiometry of the decomposition reaction was determined by the analysis of the reaction intermediates and products. The order of decreasing thermal stability, as well as order of temperature of massloss initiation, seems to be: [RuCl(NH3)5]Cl2>[RuBr(NH3)5]Br2>[RuI(NH3)5]I2 for monohalogeno complexes, and cis-[RuCl2(NH3)4]Cl>cis-[RuBr2(NH3)4]Br>cis-[RuI2(NH3)4]I for dihalogeno complexes. Trihalogenotriammine complexes were observed as intermediates for the cases of dichloro and bromo complexes, a simple decomposition reaction was found for the iodo complexes, and then the activation energy for the following reaction was determined: [RuI(NH3)5]I2→RuI3+5NH3, Ea=41.9 kcal/mol and cis-[RuI2(NH3)4]I→RuI3+4NH3, Ea=27.8 kcal/mol. The reaction mechanisms may be summarized as follows: (...