Kinetic study on the solid state reactions of transition metal complexes recently carried out in our laboratories are briefly reviewed. In the pyridine dissociation reactions of four series of analogous pyridine adducts, bis(N-methyl-substituted salicylideneaminato)bis(pyridine)nickel (II), [N,N‘ethylenebis(salicylideneaminato)](pyridine)-alkylcobalt(III), [N,N‘-ethylenebis(salicylidene-aminato](substituted pyridine)cobalt(II) and bis(2,3-alkanedionedioximato)bis(pyridine)cobalt(III) iodides, it has been found that the electronic and steric nature of the substituents affects the thermal reactivity of these adducts and Hammett‘s or Taft’s relation holds for these pyridine dissociation reactions. On the other hand, in the ligand dissociation reactions with concomitant intramolecular redox reactions of the cobalt(III) complexes, [N,N‘-ethylenebis(5-substituted-salicylideneaminato)]nitrosylcobalt(III), pentaminenitrosylcobalt(III) chloride, [N,N‘-ethylenebis(salicylideneaminato)](2,4-alkanedionato)cobalt(III), and μ-peroxo-bis[N,N‘-ethylenebis(3-sub-stitutedsalicylideneaminato)cobalt(III)], the chemical nature of the substituents exerts almost no effect on the reactivity of these complexes. It has been found that the activation entropy is the major factor determining the relative stability of the analogous complexes, while the activation enthalpy is closely related to the reducing power of the reducing ligands.