The radical ClO32–, prepared by the action of γ-rays on calcium carbonate doped with chlorate, has been identified by its e.s.r. spectrum. It has also been identified, with ClO3, as a primary radiation damage product in γ-irradiated KClO3. The principal values of the g-tensor for ClO32– in calcite are g∥= 2·0121, g⊥= 2·0105, and those of the A-tensor are a∥= 127 gauss, a⊥= 106 gauss. For ClO3 in KClO3 the principal values of the g-tensor are g∥= 2·0030, g⊥= 2·0083, and those of the A-tensor are a∥= 176 gauss, a⊥= 122 gauss. The structures, mechanisms of formation, and stabilities of these centres are discussed.