At around 3 pm, upon opening a bag in an agrochemicals plant employing 320 people, a technician noted heating of “technical mancozeb” used as a fungicide against mildew and raised the alarm. His overalls were slightly burned. At around 3.30 pm, the smell of mercaptan invaded the workshop. Armed with a fire hose cabinet, a control officer then created a water curtain. This plant health product was not manufactured on site, but came from a supplier located in another region. The next day at around 8.50 am, the external emergency plan was activated without evacuation measures or interrupting the traffic on the Motorway A6. Several bags out of the 300 big-bags of 500 kg stored for 15 days showed signs of self-ignition (up to 120 °C). The plant’s safety department isolated the bags in the establishment’s yard, in the vicinity of the polluted water retention basin. Around 60 firefighters assisted by the plant’s employees installed water curtains to prevent possible toxic emission consequences. The Prefecture activated a crisis unit at 9.15 am. A chemical emergency squad estimated the situation in the storage aisle using a thermal imaging camera. No toxic releases (H2S) were detected. At around 3 pm, 40 firefighters wearing SCBA (Self Contained Breathing Apparatus) neutralised the bags by progressive immersion in containers supplied with water by 10 nozzles. The Prefecture’s crisis unit was deactivated at around 5.50 pm, however on-call people remained on alert. Rounds were carried out during the night. On 23/09, the external emergency plan was deactivated at 11.30 am but the internal emergency plan was maintained. On 25/09, treatment of all of the bags ended at around 4.30 am and the internal emergency plan was lifted. An employee exposed to the chemical was hospitalised for less than 24 hours. The effluents from the immersion treatment (1,400 m³) were incinerated as quickly as possible. The sludge from the treatment, which was now totally inert, was recovered and treated by the fungicide supplier. The accident was reported in the press. An enquiry was carried out to determine the causes (stability of the mancozeb, use of liquid manganese sulphate instead of in crystallised form, packaging in single envelop big-bags and not in an internal polyethlyene sachet?). The self-heating of the mancozeb is irreversible above 60 °C and leads to rapid decomposition after 136 °C with the formation of CS2, SO2, and H2SO4. The dispersion of smoke containing CS2 generated concentrations lower than 400 µg/m³ in the nearest inhabited areas, which complies with the WHO’s guide value (100 µg/m³) under measurement campaign conditions. The manufacturer-supplier who said that it had noted an identical problem the week before on another stock, took samples on the bags for exothermicity analyses carried out in 2 foreign laboratories. This event was clearly identified in the safety report, however this analysis had to be revised in order to take the latest information into account.