The rate constants of the reactions OH + OH- reversible arrow O- + H2O (k(1), k(2)) and OD + OD- reversible arrow O- + D2O (k’(1), k’(2)) have been measured by pulse radiolysis in the forward and reverse directions. At 22 degrees C, the bimolecular rate constants (in mol(-1) L s(-1)) are k(1) = (6.3 +/- 1.3) x 10(9) and k(2) = (1 +/- 0.2) x 10(6) in H2O and k’(1) = (5 +/- 1) x 10(9) and k’(2) = (3.5 +/- 0.7) x 10(5) in D2O. The pks of the OH and OD radicals were calculated in nonequilibrium conditions by the ratio of the rate constants k(1) and k(2). Experiments were also performed using the same chemical system to determine the pK when the equilibrium between OH and O- or OD and O- is established. The two methods give similar results with pK(OH) = 12.0 +/- 0.2, in good agreement with the literature, and pK(OD) = 12.6 +/- 0.2. The OD radical is a weaker acid in D2O than the OH radical in H2O. The isotope effect on the equilibrium constant is mainly due to reverse reaction 2, which is 3 times slower in D2O than in H2O.