Thermal studies have been carried out on trans-[Co(NH3)4Cl2]X · YH2O complexes (whereX=IO3, BrO3, NO3, or NO2 andY=0, 1, or 2) in an effort to find cases of trans to cis isomerization as occurs for the iodate. No evidence of isomerization was found for any of the other compounds. The complexes decompose in a series of steps and these reactions have been identified and kinetic parameters determined.