The atmospheric detectors on a hydrodesulphurisation unit of a Seveso-rated refinery indicated a hydrogen sulphide (H2S) concentration above 10 ppm around 6 am. The refinery’s internal emergency plan was activated at 6:20, regional rail traffic is suspended for 90 min and a safety perimeter is set up at the facility periphery. The unit was urgently shut down, isolated and depressurised. Surveys made it possible to identify as the source of this leak a 250-mm pipeline carrying a mix of hydrogen and H2S at 31°C under 42 bar perforated in a 100-mm diameter elbow. The site’s response teams inerted and insulated the leak using solid joints. No adverse odour was detected in the towns located downwind. Rail traffic was re-authorised at 7:35 am and the emergency plan is lifted at 9 am. The inspection authorities for classified facilities and neighbours were informed. The unit was kept offline until the exact causes of this leak could be determined. Initial hypotheses pointed to pipe damage from erosion-corrosion. The pipe’s mechanical damage by H2S in a humid environment had apparently weakened the material due to spalling; also, an abnormal quantity of water in a biphasic state containing a heavy load of dissolved ammonium hydrogen sulphide salts may have been responsible for the chemical attack. The cost of operating losses was assessed at 50 million a month. An inspection of the elbow thicknesses conducted in 2004 and 2007 had failed to expose specific problems with this elbow, whose life cycle had been estimated at another 30 years.