Diphosgene is an organic chemical compound with the formula ClCO₂CCl₃. This colorless liquid is a valuable reagent in the synthesis of organic compounds. Diphosgene is related to phosgene and has comparable toxicity, but is more conveniently handled because it is a liquid, whereas phosgene is a gas.

Diphosgene is prepared by radical chlorination of methyl chloroformate under UV light:

Another method is the radical chlorination of methyl formate:

Diphosgene converts to phosgene upon heating or upon catalysis with charcoal. It is thus useful for reactions traditionally relying on phosgene. For example, it convert amines into isocyanates, secondary amines into carbamoyl chlorides, carboxylic acids into acid chlorides, and formamides into isocyanides. Diphosgene serves as a source of two equivalents of phosgene:

With α-amino acids diphosgene gives the acid chloride-isocyanates, OCNCHRCOCl, or N-carboxy-amino acid anhydrides depending on the conditions.

It hydrolyzes to release HCl in humid air.

Diphosgene is used in some laboratory preparations because it is easier to handle than phosgene.

Diphosgene was originally developed as a pulmonary agent for chemical warfare, a few months after the first use of phosgene. It was used as a poison gas in artillery shells by Germany during World War I. The first recorded battlefield use was in May 1916. German forces preferred diphosgene, liquid at room temperature, because it was easier to handle than gaseous phosgene. Diphosgene was developed because the vapors could destroy the filters of the gas masks in use at the time.