Dichlorine pentoxide is a hypothetical chlorine oxide with a chemical formula Cl<sub>2</sub>O<sub>5</sub>. The most stable configuration of dichlorine pentoxide is unknown, but theory predicts that the perchloryl/chloride peroxide structure would be the most stable among various isomers, such as the anhydride of chloric acid or the chlorous acid/perchloric acid mixed anhydride.