The epoxidation of a number of cyclic and linear terminal alkenes by an O-2-O-3 mixture at room temperatures catalysed by Fe(TMP)CI is reported. It appears that the alkene forms a primary ozonide which rearranges to a secondary ozonide, and it is the latter which, when catalysed by Fe(TMP)Cl, affects the subsequent alkene epoxidation. A number of other metal complexes were also tested as catalysts, but Fe(TMP)Cl was the most effective.