The rate of oxygen exchange between 0 ' 8 enriched CrOd2-, HCr04-, Cr2072-and solvent water has been measured as a function of pH and total chromium(V1) concentration. The exchange can be explained in terms of the dimerization equilibria HCrOa-+ C r 0 4 2 -e Cr2072-+ OH-and 2HCr04-Cr2Ol2-+ Hz0 except a t high pH, where slow pH-independent exchange is observed. Rate constants for the reactions of the dimerization equilibria have been evaluated from the exchange results, and are in agreement with values obtained by a variety of other methods.