Dichlorine hexoxide is the chemical compound with the molecular formula Cl₂O₆ or O₂Cl−O−ClO₃, which is correct for its gaseous state. However, in liquid or solid form, this chlorine oxide ionizes into the dark red ionic compound chloryl perchlorate or dioxochloronium(V) perchlorate [ClO₂]⁺[ClO₄]⁻, which may be thought of as the mixed anhydride of chloric and perchloric acids. This compound is a notable perchlorating agent.

It was originally reported to exist as the monomeric chlorine trioxide ClO₃ in gas phase, but was later shown to remain an oxygen-bridged dimer after evaporation and until thermal decomposition into chlorine perchlorate, Cl₂O₄, and oxygen. The compound ClO₃ was then rediscovered.

It is a dark red fuming liquid at room temperature that crystallizes as a red ionic compound, chloryl perchlorate, [ClO₂]⁺[ClO₄]⁻. The red color shows the presence of chloryl ions. Thus, chlorine's formal oxidation state in this compound remains a mixture of chlorine(V) and chlorine(VII) both in the gas phase and when condensed; however by breaking one oxygen-chlorine bond some electron density does shifts towards the chlorine(VII).

Cl₂O₆ is diamagnetic and is a very strong oxidizing agent. Although stable at room temperature, it explodes violently on contact with organic compounds It is a strong dehydrating agent:

Many reactions involving Cl₂O₆ reflect its ionic structure, [ClO₂]⁺[ClO₄]⁻, including the following:

It reacts with gold to produce the chloryl salt [ClO₂]⁺[Au(ClO₄)₄]⁻:

Several other transition metal perchlorate complexes are prepared using dichlorine hexoxide.

Nevertheless, it can also react as a source of the ClO₃ radical:^{[citation needed]}