Solid state thermal interconversion of [Co(NH3)(5)NO2]X-2 (nitro isomer) and [Co(NH3)(5)ONO]X-2 (nitrito isomer) complexes (X = Cl-, Br-, I-, PF6-) was studied by DSC at several heating rates. The isomerization of pure sample of both isomers leads to an equilibrium stable state in which nitro and nitrito isomers are major and minor components, respectively. Change cif the counterion affects thermodynamic parameters of the isomerization reaction, but the nitro isomer is always more stable than the nitrito one. The equilibrium constant (nitro to nitrito molar ratio) increases in the series Cl- approximate to Br- < PF6- Br- > PF6(-) > I-. The negative activation entropy supports an associative mechanism with a seven coordinate transition state in solid state. (C) 2013 Elsevier B.V. All rights reserved.