Fourier-transform ion cyclotron resonance mass spectrometry has been used to study the gas-phase oxidation of benzene mediated by "bare" MO(+) cations (M = Sc, Ti, V, Cr, Mn, Co, and Ni). Oxidation reactions by the oxides of Sc, Ti, and V were not observed, consistent with the stability of the MO(+) bond for the early-transition-metal oxides, and only condensation products, i.e., MO(C6H6)(+), were formed. For Cr through Ni oxide cations, the most abundant process corresponds to the exothermic conversion of benzene to phenol. All oxidation reactions are kinetically efficient; i.e., k(f)/k(C) approximate to 1. The reactivity of each metal oxide cation is examined, and mechanistic details for various processes have been uncovered based an mass spectrometric evidence and labeling studies by using [D-6]benzene and [1,3,5-D-3]benzene. The operation of an intramolecular isotope effect of k(H)/k(D) = 3.7 for MnO+ was evaluated from the relative product intensities of MnOH+ and MnOD+ generated from the reaction with [1,3,5-D-3]benzene. BDE-(Mn+-OH) = 82 +/- 7 kcal/mol has been derived from H-atom-transfer reactions.