On 11/06 in an agrochemicals production plant, located on a multi-operator site, a leak occurred on the sealing of the agitator while a reaction using Cl2 was underway in a reactor. After 10 am, although the reaction was not finished, the content (35 °C) was transferred into a buffer tank in order to repair it. The buffer capacity was electrically heat traced (external resistance) to maintain the reaction mixture up to 70 °C. On 12/06, at around 3.45 am, a vinegar smell was noticed near the buffer. The technicians then noticed a leak on the manhole and noted that the pressure was 4 bar. The on-call technician was contacted. At 4.15 am, the pressure relief valve calibrated at 4.2 bar opened under the action of the pressure and allowed gases to escape forming a cloud above 2 buildings. The cloud was stationary as there was no wind. Once the dome of the tank had been sprayed, the internal pressure, including the discharges, dropped. The electrical heat tracing was stopped: the tank’s external temperature was then 53 °C. At 4.50 am, the area around the 2 buildings was beaconed off to restrict its access. The site’s internal emergency plan was activated at 6.25 am, and the leak was stopped before the staff arrived at 6.45 am. The external emergency services were alerted, cleaning staff were evacuated, and site access was restricted to staff who were essential to crisis management and to maintaining the safety of the other establishments. Deliveries were stopped. As the emissions persisted, the reaction intermediates contained in a buffer tank (potassium thiocyanate, potassium sulfochloride, acetic acid, H2O, HCl, CO2) were transferred at 45 °C to the reactor. The reaction mass was flooded with water to cool it and the agitator’s sealing was sprayed. At 10.30 am, the internal emergency plan was lifted and deliveries were resumed. The content of the reactor was to be subsequently destroyed. The cooling undertaken since 4.15 am only reduced the temperature of the gaseous phase and not that of the reaction mixture. The electrical heat tracing would have brought the 7 t of liquid from 35 °C to 45 °C, in 17 hours. The increase in pressure could not possibly have been related to the boiling of acetic acid (118 °C). This increase in temperature would have therefore caused vaporisation of the dissolved gases, and acetic acid, HCl and CO2 would have been emitted. The resumption of this reaction in half-batches was not considered.