[en] Radiolysis of aqueous solution of di and trivalent cobalt with 1:2 (bis)carboxy-methylaminodiethyltetraacetic acid (EGTA) was investigated, both in absence and in presence of oxygen. The [Co(II) + EGTA] complex was prepared by mixing a known volume of 8x10^-2M cobalt chloride with the same volume of 8x10^-2M, 1.6x10^-1M or 4x10^-1M EGTA. The pH of the solutions was then adjusted using NaOH free carbonate. The [Co(III) + EGTA] complex was prepared by oxidising the [Co(II) + EGTA] complex at pH 8 with few drops of H₂O₂. Irradiations were carried out using γ-rays from ⁶⁰Co at a dose rate 1.7x10³ Rad/min. A radiolytic mechanism has been proposed. It has been shown that the degradation at the ligand of the chelate is due to OH only. (T.G.)