The stoichiometry and stability of the complexes formed between Co (II) and Co (III) on one side and nitrilotriacetic acid on the other side was spectrophotometrically investigated. The chelation of the hydrolysed state of Co (III) with the considered acid was also studied. The behaviour of the Co (Il)-complex on irradiation with cobalt-60 γ-rays was followed spectrophotometrically.