The chemistry of aqeous ozone, O3, and ozonide, O3−, is of great interest from a technological, environmental and scientific point of view. The literature about their aqueous chemistry is extensive, the reaction mechanisms are still not well understood. The ozonide anion is a free radical that is too unstable in aqueous media to be studied by classical means. Some properties of the aqueous ozonide radical ion have been elucidated earlier by means of the pulse radiolysis technique. The OH− catalyzed chain decomposition of aqueous ozone, 2O3→3O2, has not yet been rationalized in terms of a detailed mechanism. We have investigated the reaction mechanisms using pulse radiolysis in combination with a high pressure cell, rapid mixing and numerical simulation of the experiments. This study makes it possible to give a detailed description of the reaction mechanisms in terms of uni- and bimolecular reactions and provides kinetic data sufficient for computer simulations of aqueous O3/O3− chemistry.