The reaction of perchloro-(3, 4-dimethylenecyclobutene) (I) with molecular oxygen in the solutions of carbon tetrachloride, benzene and ether was investigated. The identified products were phosgene, perchloro-(dimethylenesuccinic) acid (IIa), its dichloride, its anhydride, perchloro-(2, 3-dimethylenecyclobutanone) (IV) and 2, 2, 3′, 3′, 4-pentachloro-3-methyleneglutaric acid (VIII) in carbon tetrachloride