Dichlorine trioxide, Cl2O3, is a chlorine oxide. It is a dark brown solid discovered in 1967 which is explosive even below 0 °C. It is formed by the low-temperature photolysis of ClO2 and is formed along with Cl2O6, Cl2 and O2. Its structure is believed to be OCl-ClO2 with possible isomers such as Cl-O-ClO2. It is the theoretical anhydride of chlorous acid.