An exothermic reaction involving diethanolamine and thionyl chloride in solvent phase, initiated 16 hours earlier, yielded chlorhydrate and a gaseous effluent neutralised in a washer after exposure in a glass column. Following a whistling noise, the column burst at 9 am. A toxic cloud (composed of HCl/SO2) drifted onto a street adjacent to the plant, where residents had to be evacuated. The internal emergency plan was activated: 13 people suffered from intoxication and were hospitalised 15 min later. The unit had been automated for a year. The flow/reagent/agitation servo control mechanism, planned at the outset, had never been installed. One of the 4 technicians well versed in plant operations acknowledged an (agitation) alarm at the beginning of the reaction. The stirrer had remained turned off. The accident occurred upon manually restarting agitation after an electrical outage in the workshop.