Abstract 1. From the solubility of cobalt (III) nitroammine complexes the thermodynamic stability was found to decrease in the order, chloride>nitrate>bromide>perchlorate. 2. The free energy of solution of cis-dinitrotetramminecobalt(III) chloride is lower than that of the trans chloride. This fact is attributed to the abnormally large value of the entropy of solution of the former compared with the latter. 3. Between the free energy of formation of cobalt (III) nitroammine complexes and the number of coordinated nitro-groups there exists a linear relationship.