The carbonate radical, CO3-, may be generated by the reaction of carbonate or bicarbonate ions with hydroxyl radicals. The flash photolysis of hydrogen peroxide solutions and the pulse radiolysis of water are utilized as sources of hydroxyl radicals from which it is found that the basic form of the hydroxyl radical, O-, contributes little to the production of CO3-, even up to pH 14. Reactions of CO3- with hydrogen peroxide (k = 8 × 105 M-1 sec-1) and with HO2- (k = 5.6 × 107 M-1 sec-1) are observed. The carbonate radical and the perhydroxyl radical ion, O2-, are formed in equimolar concentrations in the flash photolysis of oxygen-saturated carbonate solutions, and from this, the extinction coefficient of O2- is found to be 1850 ± 200 M-1 cm-1 by comparison with the better known extinction coefficient of CO3-. The product of the reaction CO3- + O2- is assumed to be CO5- having a half-life time of several seconds and εCO52-2600 = 410 M-1 cm-1.