A pulse radiolysis study which uses a flow system to irradiate mixtures of H2CO3 and HCO3- within 50 ms of their formation has demonstrated that the carbonate radical is a strong acid, pK(a) < 0, contrary to published reports of high pK(a)s. The rate constant for reaction of the hydroxyl radical with H2CO3 is 7 x 10(4) M-1 s(-1) at about 5 degrees C.