The kinetics of the reaction, via an oxygen-bound intermediate, of [Cu-2(1)(NO2-XYL)][ClO4](2). CH3-CN to [Cu-2(NO2-XYL-O-)(OH)](2+), where the bridging arene is hydroxylated, have been examined with use of resonance Raman spectroscopy. A resonance Raman peak characteristic of peroxide bound in a side-on, mu-eta(2):eta(2) geometry is observed upon oxygenation of [Cu-2(1)(NO2-XYL)] for both intramolecularly and intermolecularly bridged complexes. The decay of the intramolecularly bridged peroxide stretch at similar to 750 cm(-1) and the growth of the phenolate stretch of the product at 1320 cm(-1) were monitored over time with use of an excitation wavelength of 406.9 nm. Both the decay of the peroxide stretch and the growth of the phenolate stretch were found to be first order, and the rate constants are consistent, within experimental error, with the peroxide intermediate reacting directly to form the hydroxylated product. The possibility of an unobservable amount of a bis-mu-oxo isomer which is in rapid equilibrium with the side-on peroxide species, and that is responsible for the hydroxylation reaction, is considered. An upper limit for the concentration of the bis-mu-oxo isomer in a solution of [CU2(NO2-XYL)(O-2)](2+) was determined. This gives the lower limit for its rate of reaction to form the phenolate product, which is approximately 1000 times faster than the decay of the peroxide intermediate. A comparison of the reactivities of the side-on peroxide and bis-mu-oxo isomers with respect to electrophilic aromatic substitution is made by using frontier molecular orbital theory. This correlation, in conjunction with the estimated, relative rates of reaction for the two isomers to form phenolate product, leads to a molecular mechanism in which the side-on peroxide isomer is likely to be the reactive oxygen intermediate in these systems.