Thermal annealing reactions of 57Co formed by the EC and β+-decay of the 57Ni-labeled hexamminenickel(II) halide complexes were investigated in detail by means of ion-exchange techniques. When the annealing is carried out at high temperatures, the chemical behavior of 57Co is remarkably affected by the thermal decomposition of the parent and daughter complexes. In the intrinsic annealing, however, the annealing curves have characteristic shapes and the plateau yields of 57Co(NH3)63+ ion are independent of the temperature, depending on the nature of the outer-sphere anion. The unusual oxidation reaction of the recoil 57Co(II) species is explained by the action of the crystal defects in the near-neighborhood of the recoil site.