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

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. Diphosgene was developed because the vapors could destroy the filters in gas masks in use at the time.

Diphogene is commonly available and therefore is a credible threat as a terrorist weapon. When its ability to penetrate a carbon based protective mask is considered, it becomes even more attractive to the terrorist.

Clinically Diphosgene behaves like phosgene, it has the ability to caused delayed pulmonary edema. Respiratory effects begin to occur at 1 to 10 ppm.

Decontamination with soap and water is recommended. Operations teams are advised to don level B or A protection when handling casualties. Filter type masks may be rendered useless and are not recommended.