Chlorine peroxide (also known as dichlorine dioxide or ClO dimer) is a molecular compound with formula ClOOCl.

[1] Chemically, it is a dimer of the chlorine monoxide radical (ClO·).

[2] Chlorine peroxide catalytically converts ozone into oxygen when it is irradiated by ultraviolet light.

[3] Difluorodichloromethane (CF2Cl2) can also act as a source of chlorine atoms for the formation of the peroxide.

It also absorbs longer wavelengths up to 350 nm to a lesser extent.