Chloride ions are efficient catalysts for the synthesis of phosgene from carbon monoxide and elemental chlorine at room temperature and atmospheric pressure. Control experiments rule out a radical mechanism and highlight the role of triethylmethylammonium trichloride, [NEt3Me][Cl3], as active species. In the catalytic reaction, commercially available [NEt3Me]Cl reacts with Cl2 to form [NEt3Me][Cl3], enabling the insertion of CO into an activated Cl─Cl bond with a calculated energy barrier of 56.9 to 77.6 kJ mol−1. As [NEt3Me]Cl is also a useful chlorine storage medium, it could serve as a catalyst for phosgene production and as chlorine storage in a combined industrial process.