Aqueous solutions of Co(III)-ethylenediamine tetraacetate (C) at different pH were irradiated with X-rays both in the presence and absence of dissolved oxygen. Radiolytic decomposition of the metal chelate and the formation of hydrogen peroxide and formaldehyde were determined. The radiolysis was found to be initiated by attack both at the ligand and at the central metal ion of the metal chelate by the free radicals derived from water. A detailed radio- lytic mechanism has been discussed. From competition kinetic studies with methanol in neutral medium the rate constant, k(OH + C) was found to be 5.3 × 108M−1sec−1.