Trioxidane (systematically named dihydrogen trioxide,), also called hydrogen trioxide is an inorganic compound with the chemical formula  (can be written as  or ). It is one of the unstable hydrogen polyoxides.  In aqueous solutions, trioxidane decomposes to form water and singlet oxygen:
The reverse reaction, the addition of singlet oxygen to water, typically does not occur in part due to the scarcity of singlet oxygen. In biological systems, however, ozone is known to be generated from singlet oxygen, and the presumed mechanism is an antibody-catalyzed production of trioxidane from singlet oxygen.