Experimental and theoretical studies of O3 decomposition are reported which resolve kinetic mechanisms for isotopic fractionations in O-O2-O3 chemistry. The thermal gas phase decomposition of O3 at high temperatures (90 ¿C, 110 ¿C) produces isotopically heavy O2 with respect to precursor O3 with a non-mass-dependent fractionation pattern (Δ17O≠0.5Δ18O). At 90 ¿C, a noticeable component of heterogeneous decomposition is present, but is insignificant at 110 ¿C. The product O2 from O3 photolysis by visible (532 nm) and UV light is depleted in the heavy isotopes, opposite from thermal decomposition. These well controlled experiments permit a quantitative kinetic analysis. A one-box, time-dependent model was used to simulate the evolution of the isotopic composition of the O2 and O3. The comparison of thermal and photolysis experiments indicates that the initial decomposition step is most likely responsible for the observed isotopic fractionation. Based upon detailed kinetic model calculations, the roles of isotope exchange reactions, initial O3 isotopic distribution, and various branching ratios of isotopic reactions involved in the isotopic fractionation have been examined. The possible importance of these conclusions for understanding stratospheric isotopically heavy O3, and possible implications for non-mass-dependent isotopic fractionations and chemical kinetic theory are discussed.