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The stabilities and reactivities of cluster ions generated from the fast-atom bombardment (FAB) or the direct laser vaporization (DLV) of the Group 13 metal oxides (Al2O3, Ga2O3, and In2O3) were examined by mass spectrometry. The nascent cluster ion distributions, fragmentations, and reactions were studied. The observed patterns of stability and reactivity were compared with the structures and heats of formation calculated from theoretical studies of aluminum oxide cluster ions using MNDO, Xα, and Born–Mayer pair potentials. The method of production of the metal oxide cluster ions, whether by FAB, DLV, or through the reaction of sputtered bare metal cluster ions with oxygen, had little influence on the abundance distribution observed. In agreement with the known M–O binding energies, a trend of increasing cluster oxidation state was observed in the abundance distributions of the cluster ions for decreasing metal atom z value. Dissociation of the oxide cluster ions occurred through the loss of particularly stable neutral fragments which together with theoretical considerations suggest a preference for particular parent cluster stoichiometries. Although gallium oxide and indium oxide cluster ions exhibited little reactivity toward oxidation agents, the aluminum oxide ions reacted readily with most oxidants. ‘‘Oxygen saturation’’ effects were observed for the ions with 2 and 3 aluminum atoms. These saturation effects could be explained by the formation of structures in which the Al atoms are three-fold planar coordinated.