Dichlorine heptoxide is the chemical compound with the formula Cl₂O₇. This chlorine oxide is the anhydride of perchloric acid. It is produced by the careful distillation of perchloric acid in the presence of the dehydrating agent phosphorus pentoxide:

Cl₂O₇ can be distilled off from the mixture.

It may also be formed by illumination of mixtures of chlorine and ozone with blue light. It slowly hydrolyzes back to perchloric acid.

Cl₂O₇ is an endergonic molecule, meaning it is intrinsically unstable, decomposing to its constituent elements with release of energy:

Dichlorine heptoxide is a covalent compound consisting of two ClO₃ groups linked by an oxygen atom. It has an overall bent molecular geometry (C₂ symmetry), with a Cl−O−Cl angle of 118.6°. The chlorine–oxygen bond lengths are 1.709 Å in the central region and 1.405 Å within each ClO₃ cluster. In this compound, chlorine exists in its highest formal oxidation state of +7.

Dichlorine heptoxide reacts with primary and secondary amines in carbon tetrachloride solution to yield perchloric amides:

It also reacts with alkenes to give alkyl perchlorates. For example, it reacts with propene in carbon tetrachloride solution to yield isopropyl perchlorate and 1-chloro-2-propyl perchlorate.

Dichlorine heptoxide reacts with alcohols to form alkyl perchlorates.