Nearly 1 kg of phosgene (COCl2) was released during 90 seconds through the flue of the hexamethylene diisocyanate (HDI) production workshop inside a chemical plant. The analyser alarms at the scrubbing column outlet and the site’s environmental detector (threshold > 1.5 ppm) were both tripped, causing the equipment’s confinement valves to close and flushing ammonia into the flue in order to neutralise the phosgene and disseminate a gas alert to the entire chemical platform. No human or environmental consequences were recorded. The alert was lifted 45 min after activation. The accident occurred at the time of an operation intended to remove phosgene preliminary to the inspection of a phosgene tank, made necessary by the observation of fouling on the column boiler used to separate hydrochloric acid (HCl). This step consisted of treating the solvent (monochlorobenzene) loaded with COCl2 via two separation columns (HCl and COCl2) and then channelling the gaseous phosgene through the workshop’s venting circuits and the 2 soda scrubbing columns. A pressure drop in the second separation column had caused, via the venting circuit, reflux of a phosgene-nitrogen inerting mix from another workshop tank, whose vent was being treated by the scrubbing columns. The presence of nitrogen significantly lowered absorption output by the treatment systems (respectively from 500 kg/h and 2,000 kg/h to 360 kg/h and 1,360 kg/h) and caused the phosgene discharge into the flue despite a final scrubbing with ammonia.