Hydration of aluminum oxide anion clusters was studied in the gas phase using an ion trap secondary ion mass spectrometer. Hydration of both AlO2- and Al2O4H- occurred by the consecutive addition of two H2O molecules. For hydration of AlO2-, the rate constants for addition of the first and second water molecules are 4 x 10(-11) and 4 x 10(-10) cm(3) molecule(-1) s(-1), respectively. The first and second hydration rate constants for Al2O4H- are 2 x 10(-9) and 8 x 10(-10) cm(3) molecule(-1) s(-1), respectively. A comparison of the experimental rate constants to the theoretical rate constants reveals that addition of the first H2O to AlO2- is only 2% efficient, whereas addition of the first H2O to Al2O4H- is 100% efficient. Ab initio calculations were performed to assist in the interpretation of the kinetic results. Reaction mechanisms and energetics for the hydration of the AlO2- system were calculated using the HF/6-311+G(d(Al),p), B3LYP/6-31+G(d), B3LYP/6-311+G-(2d,p), B3LYP/6-311+G(3d2f,2p), and MP2/6-311+G(2d,p) levels of theory. Calculations on the hydration of the Al2O4H- system were performed using the B3LYP/6-311+G(2d,p) level of theory. Ab initio results revealed that the addition of the first and second waters, for both the AlO2- and Al2O4H-systems, results in the formation of four-membered transition states, with simultaneous Al-O bond formation and proton transfer. However, a significant later transition state is observed, with respect to the Al-O and H-O bond lengths, for the addition of the second water molecule in the Al2O4H-system. A comparison of the reaction mechanisms and energetics was not sufficient to account for the 2 orders of magnitude difference in rate constants; however, the reactivity differences do correlate with the dipole moment of the aluminum oxide anions, which may serve to preorient the incoming water molecule, thus enhancing the reaction rate.