Ozone decomposition was investigated under reaction conditions by using Raman Spectroscopy on a supported manganese oxide catalyst. An adsorbed species with a Raman signal at 884 cm(-1) was observed during reaction, and was identified as a:peroxide species by a combination Of in situ Raman spectroscopy and O-18 isotopic substitution measurements. The observed isotope shift nu(O-18)/nu(O-16) value of this adsorbed species was 0.946, in excellent agreement with the ratio calculated for a peroxide species (0.943) by using a harmonic oscillator model. Ab initio molecular orbital calculations confirmed this assignment, using Mn(OH)(4)(O-2)(+) as a model compound for the adsorbed species. The reaction mechanism of ozone decomposition was elucidated with carefully designed experiments, including isotopic substitution. The reaction sequence involved dissociative adsorption of ozone to form an oxygen molecule and an atomic oxygen species, reaction of the atomic species with gaseous ozone to form an adsorbed peroxide species and gas-phase oxygen, and decomposition of the peroxide intermediate and desorption of molecular oxygen.