A 40 l ammonia leak occurred in a synthetic rubber plant. The gas alarm and the internal emergency plan were activated, the staff were kept inside and access to the site was closed. The internal emergency services intervened with protective clothing and SCBA (self-contained breathing apparatus). The leak was detected on a flange which supplied the polymerisation reactors with ammonia at under 7 bar. The ammonia solution was injected at the end of the reaction in order to stop it. It was propelled through the line by a pump located at the top, also used for unloading lorries. The leaky flange was flanked by 2 valves, one regulating the amount of ammonia downstream and the other operating as an on-off device upstream. The leak was located and brought under control 30 minutes later. The NH3 concentration in the workshop was close to 40 ppm, and the internal emergency plan was lifted. The position of the seal on the flange was correct but part of it was detached, thereby causing the leak of ammonia in the workshop. The line for the leaky flange was correctly attached by mounts and had not been deformed. The flange in question and another one nearby by were both covered with a plastic protective strip and secured by mass-produced flexible strapping. The implementation of internal emergency response procedures and the rapid control of the incident meant that the external emergency services did not need to intervene. No incident of this type had been noted since the installation of these seals following prohibition of the use of asbestos in 1999. The operator has replaced the damaged seal with a new one and indicated that this malfunction would be examined within the framework of the alert procedure. An analysis of the incident with the site’s managers will need to identify the failures via a fault tree and technological or organisational barriers placed upstream and downstream of the event. The report for this analysis will have to be validated by the company and must mention the process for recording this incident in the safety management system. It will need to provide additional information about steps taken to prevent any recurrence of this incident.