A highly reactive oxide of chlorine with chemical formula OClO. This species is of importance in the chemistry of polar stratospheric ozone depletion, as it is a product of the reaction of ClO with BrO. Its strong, structured absorption spectrum in the near-UV–visible region makes it extremely susceptible to destruction by photolysis, but also makes possible its detection via UV absorption in the atmosphere. Its detection in the antarctic ozone hole was one of the key discoveries that pointed to chlorine chemistry being the major cause of polar stratospheric mountain-wave cloud depletion. This compound should not be confused with the isomeric ClOO radical, which is thermally unstable (dissociating to Cl and O2).