Mono- and bis-adducts of benzene of the type M(C6H6)(1,2)(+) have been generated in the gas phase with first-, second-, and third-row transition metal cations, and their reactivities toward molecular oxygen have been measured using an inductively coupled plasma/selected-ion flow tube (ICP/SIFT) tandem mass spectrometer. Trends in reactivity were identified for the metal cation adducts across and down the periodic table. The intrinsic effect of the benzene ligand(s) on the reactivity of the bare metal cations was determined through comparisons with earlier results obtained for the bare metal ions. Molecular oxygen activation by early transition metal cations is preserved in the presence of one benzene ligand but almost disappears in the presence of two benzene molecules. The rate coefficients for O-2 addition to late transition metal-benzene adducts are enhanced by up to 4 orders of magnitude in the presence of benzene. Thermochemical information is derived from the occurrence or absence of ligand-switching or metal-abstraction reactions. Oxidation of the benzene ligand itself was observed to be mediated by the transition metal cations V+, Cr+, Fe+, Co+, and Re+ and appeared to result from simultaneous activation of C-H, C-C, and O-O bonds. Catalytic oxidation of the benzene ring observed in the presence of Fe+, Cr+, and Co+ is postulated to result in the production of catechol.