An ammonia leak occurred in the machinery room housing the refrigeration systems of a warehouse located in an industrial area. Activated by a sensor (500 ppm), the ammonia detection system activated visual and audible alarms at 5:06 a.m. and notified the on-call technician by phone at 5:08 a.m. The warehouse was closed at 5:11 a.m. and the 50 employees present evacuated the site. The situation returned to normal at 5:26 a.m. (ammonia concentration below the threshold). The on-call technician informed the warehouse’s technical manager at 5:27 a.m. The logistics manager notified the fire-fighters at 5:53 a.m. (as per the fire procedure, not the ammonia procedure). The fire-fighters arrived on the scene at 6:04 a.m. The on-call technician arrived one minute later and acknowledged the fault in order to stop the alarms. The site’s technical manager arrived on the scene at 6:20 a.m. and secured the facility by cutting off the power supplies to the non-ATEX equipment and turning on the ammonia exhaust systems. A compressor was then turned off and isolated by closing a valve upstream of the valves. Another compressor was turned on to resupply cold air to the store rooms. The specific cause of the incident was sought starting at 7:00 a.m. A ruptured valve was found. A three-way valve was switched to activate the second valve on the affected compressor, which was then turned back on and monitored by technicians. The valve that unexpectedly opened will be replaced. The fire-fighters made a final check and then left. The safety cordon was lifted. The gendarmes also arrived on the scene. The operating staff were reminded about the ammonia procedures and a meeting with fire-fighters was held in the following weeks.