Thermal or hydrogen peroxide initiated decomposition of acidic aqueous ozone is described as a chain process propagated by OH and O-2(-)/HO2 radicals. In the absence of H2O2 added, the chain is initiated by surface-catalyzed formation of H2O2. Surface-dependent formation of H2O2 is experimentally verified. Since homogeneous radical-radical reactions are much too inefficient to account for the termination, a heterogeneous chain termination is strongly suggested. Without addition of H2O2 no dependence of the decay on the surface-to-volume ratio could be observed. However, on initial addition of H2O2 in amounts sufficient for homogeneous initiation to prevail, the rate of O-3 decay becomes proportional to the surface of the reaction vessel. These findings strongly suggest that the decomposition of acidic aqueous O-3 is governed by initiation and termination processes occurring at the reaction-vessel surface, while its apparent appearance of a homogeneous solution-phase reaction results from the counteractive nature of these two processes.