Fe(II)-induced decomposition of 1,2,4-trioxolanes (ozonides) 1 and 2 is studied in detail. The innersphere electron transfer reduction, which is sensitive to steric effects, is proposed to be a reasonable mechanism for the peroxide decomposition, i.e., the selective generation of one of the two possible Fe(III)-complexed oxy radicals. The fate of the oxy radical species is revealed in detail by using O-18-tracer studies in the reduction of O-18-labeled ozonides 1a and 2a, in which the. ethereal oxygens are labeled by O-18 atom (O-18, 77%) and also by the reduction in the presence of (H2O)-O-18 (O-18, 10%). The results of the O-18-tracer studies are consistent with the regioselective generation of the Fe(III)-complexed oxy radical species.