β-Radiolysis of [Co(en-d4)3]Cl3・3T2O was studied under a mild condition by means of infrared spectroscopy over a period of 8 months. The spectra became complex due to the appearance of many new bands in the initial process. This is explained in terms of the exchange reaction in hydrogen isotopes between water and ethylenediamine. It was found that ethylenediamine decomposed by ～50% in 100 days, forming ammonia coordinated to a Co2+ ion and an ammonium ion. On the other hand, a Co (Ⅲ) ammine complex, acetylene and so on were not found. A scheme for the β-radiolysis is proposed
