Solid state thermal isomerization of [Co(NH3)(5)(ONO)]Cl-2 (nitrito isomer) to [Co(NH3)(5)(NO2)]Cl-2 (nitro isomer) and reverse reaction were investigated by non-isothermal differential scanning calorimetry (DSC) and found to be essentially an equilibrium process. The interconversions are accelerated at above 65degreesC and reach to equilibrium state at about 155degreesC. After establishment of the equilibrium the relative amounts of two isomers at any temperature are governed by Gibbs free energy relationship. The experimental enthalpy changes of isomerization of pure nitrito and nitro solid samples to the equilibrium state are -4.67 (+/-0.19) and 0.99 (+/-0.05) kJ mol(-1), respectively. From these values, total enthalpy change was calculated as: DeltaHdegrees = -5.66(+/-0.20) kJ mol(-1). Using Gibbs free energy relationship, equilibrium constant, total free energy and entropy changes were estimated at 60degreesC as: K = 7.72 (+/-0.8), DeltadegreesG = -5.54 (+/-0.20) kJ mol(-1) and DeltaSdegrees = -0.36 (+/-0.80) J K-1 mol(-1). An initial rate method has been developed to determine the kinetic parameters of these reactions from non-isothermal DSC data. Both nitro to nitrito and reverse reactions obey first order kinetic law in solid state. Estimated activation parameters of forward and reverse paths at 60degreesC are DeltaH(ONO)(double dagger) = 103 (+/-3) kJmol(-1), DeltaS(ONO)(double dagger) = -6(+/-1) J K-1 mol(-1), DeltaH(NO2)double dagger = 108(+/-5) kJ mol(-1) and DeltaS(NO2)(double dagger) = -3(+/-1) J K-1 mol(-1), respectively. The negative activation entropy of both directions support the intramolecular mechanism of isomerization, including formation of a seven coordinate transition state, which formerly suggested based on spectral and X-ray methods. (C) 2003 Elsevier B.V. All rights reserved.