The kinetic properties and thermal stability of [Co(4-Cl-3-OHpy)(2)Cl-2]. 2H(2)O and trans-[Ni(4-C1-3-OHpy)(4)Cl-2]. 4H(2)O were studied in a dynamic nitrogen atmosphere using TG, DTG and DSC methods. Kinetic parameters were calculated using mechanistic integral equations. For the tetrahedral cobalt(II) complex, the three-dimensional diffusion (Ginstling-Brounshtein function) D-4 mechanism governs the degradation process; for the octahedral nickel(II) complex the three-dimensional diffusion (Jander function) D-3 mechanism acts up to alpha approximate to 0.5 decomposition.