We have conducted a density functional theory investigation into the gas-phase reactivity of small gold cluster ions in the interest of understanding gold cluster reactivity in several catalytic systems. Previously unreported geometries for Au-9(-) and Au-10(-) anions are obtained and reported from geometry optimizations. Predicted values of the vertical detachment energy match well with experiment, as does a rough simulation of its ultraviolet photoelectron spectrum-we found that comparison of predicted spectra with experimental data is a more sensitive analysis of geometry differences. Several binding sites for O-2 with different energies are identified on Au-10(-), but we show that the strongest binding site and orientation is predicted by frontier orbital theory. In addition, weakly stable adsorbed states for O-2 on the anion clusters Au-9(-), Au-10(-), and Au-11(-) are predicted in agreement with frontier orbital theory. The calculated binding energies are consistent with the experimentally observed patterns in adsorption of O-2 on anionic Au clusters. The binding energy for O-2 to Au-10(-) was calculated to be 19 kcal/mol, higher than for O-2 to either Au-9(-) (4 kcal/mol) or Au-11(-) (5 kcal/mol), and the calculated O-O bond length was found to increase from its gas-phase value of 1.27 angstroms to 1.38 angstroms when adsorbed on the Au-10(-) cluster, approaching the calculated bond length of 1.41 angstroms for the gas-phase superoxide ion O-2(-).