An explosion occurred in a supply pipe of the nitration workshop in a chemical plant. The pipe contained a sulphonitric acid mixture (H2SO4 + HNO3) used for manufacturing dinitrotoluene (DNT). The internal emergency plan was activated. An employee was hospitalised for hearing problems. There was extensive material damage on the site, particularly due to projections of metal fragments: the roof was partially ripped off, the workshop’s pipes were deformed or ripped off (6 m of the H2SO4 pipe were destroyed), equipment on that floor was perforated by fragments and the cladding of the neighbouring workshop was damaged. Three workshops were shut down as a precaution to check any potentially damaged equipment. No environmental consequences were reported. The installation had been secured (H2SO4 and HNO3 supply stopped) 5 minutes before the explosion and following a ‘low flow’ alert on the H2SO4 supply. When attempting to restart it one minute later, the technician activated the emergency stop for the same reason. To prevent the reactive mixture from setting in the reactor, the heating valve on the unit was opened. The detonation occurred 4 minutes later. The accident was caused by a lack of H2SO4 supply whereas the indicator light was showing that the feed pump was operating normally (presumably due to voltage produced by the adjacent electric cables). ‘Upstream negative pressure’ in the supply line due to the pump not operating caused the reactive mixture (mononitrotoluene (MNT)+ a small proportion of DNT) to be siphoned from the reactor towards the sulphonitric pipe. Contact between the HNO3 and this mixture in a confined space generated an extremely exothermic reaction and then the explosion. The failure of the H2SO4 feed pump was caused by maintenance work on the neighbouring H2SO4 recycling unit (loss of the 24 V instrumentation network on the H2SO4 pump unit). Corrective measures are being implemented by the operator: installation of a low flow safety mechanism on the supply and low pressure safety mechanism upstream of the H2SO4 manifold, improvement of the design of the sulphuric circuit, installation of a cable protected against the voltages produced on the H2SO4 pump instrumentation, and installation of a low pressure safety mechanism on the reagent intake pipes, etc.