In aqueous solutions, the aquachromyl(IV) ion, CraqO2+, disproportionates to Cr-aq(3+) and HCrO4-. The reaction exhibits second-order kinetics with an inverse [H+] dependence, -d[CraqO2+]/dt = 38.8[CraqO2+](2)[H+](-1) at 25 degreesC. The combination of the rate law and substantial kinetic isotope effect, k(H)/k(D) = 6.9, suggests a mechanism whereby a hydrogen atom is abstracted from a coordinated molecule of water or hydroxo group within a singly deprotonated transition state. The buildup of chromate is more complicated and somewhat slower than the loss of chromyl, suggesting the involvement of intermediates.