The above system has been studied at 30±0.1° and 50±0.1° with Schreinemakers' method; the equilibrium concentration of ethylenediamine varied from 0.2 to 7 M. It is found that under our experimental conditions, the solid phase in equilibrium with the solution consists of the cobalt(Ⅱ) complex, [Co(en)₃]SO₄,only.