(+)- and carbonyl-18O-labelled β phenylisobutyryl peroxides were decomposed in the presence of antimony pentachloride in light petroleum. Optically active (+)-1-phenyl-2-propyl β-phenylisobutyrate, (+)-β-phenylisobutyric acid, 2-chloro-1-phenylpropane, n-propylbenzene and carbon dioxide were obtained as main products. The carbonyl oxygen of the peroxide was retained as much as 83 ∼ 89% in the carbonyl