Chromium oxyanions having the general formula CrxOyHz- play a key role in many industrial, environmental, and analytical processes, which motivated investigations of their intrinsic reactivity. Reactions with water are perhaps the most significant, and were studied by generating CrxOyHz- in the gas phase using a quadrupole ion trap secondary ion mass spectrometer. Of the ions in the Cr1OyHz envelope (y = 2, 3, 4; z = 0, 1), only CrO2- was observed to react with H2O, producing the hydrated CrO3H2- at a slow rate (similar to0.07% of the ion-molecule collision constant at 310 K). CrO3-, CrO4-, and CrO4H- were unreactive. In contrast, Cr2O4-, Cr2O5-, and Cr2O5H2- displayed a considerable tendency to react with H2O. Cr2O4- underwent sequential reactions with H2O, initially producing Cr2O5H2- at a rate that was similar to7% efficient. Cr2O5H2- then reacted with a second H2O by addition to form Cr2O6H4- (1.8% efficient) and by OH abstraction to form Cr2O6H3- (0.6% efficient). The reactions of Cr2O5- were similar to those of Cr2O5H2-: Cr2O5- underwent addition to form Cr2O6H2- (3% efficient) and OH abstraction to form Cr2O6H- (<1% efficient). By comparison, Cr2O6- was unreactive with H2O, and in fact, no further H2O addition could be observed for any of the Cr2O6Hz- anions. Hartree-Fock ab initio calculations showed that reactive CrxOyHz- species underwent nucleophilic attack by the incoming H20 molecules, which produced an initially formed adduct in which the water 0 was bound to a Cr center. The experimental and computational studies suggested that Cr2OyHz- species that have bi- or tricoordinated Cr centers are susceptible to attack by H20; however, when the metal becomes tetracoordinate, reactivity stops. For the Cr2OyHz- anions the lowest energy structures all contained rhombic Cr2O2 rings with pendant O atoms and/or OH groups. The initially formed [Cr2Oy- + H2O] adducts underwent H rearrangement to a gem O atom to produce stable dihydroxy structures. The calculations indicated that rearrangement did not occur in the [Cr2O5H2- + H2O] adduct, possibly because the rearranged product could not accommodate the negative charge.