Around 8:30 am, two cylinders of ammonia (NH3) exploded in a metal surface treatment plant that had been shutdown for the Christmas holidays, with the personnel onsite solely performing maintenance tasks. A smell of NH3 in the vacuum oven workshop drew the Maintenance Supervisor’s attention. The two cylinders were connected to one of the two booms containing 5 cylinders each as part of the distribution system. The employees quickly brought over a tank filled with water to immerse the damaged cylinders and dissolve the remaining gas. A similar accident had already occurred during a summer downtime period on 08/18/2002 (ARIA 24897). An analysis was conducted in conjunction with the supplier. A new cylinder contains 85% liquid NH3 and 15% gaseous phase. If several cylinders are in service on the same boom and one or more are at a slightly higher temperature than the others, a liquid NH3 transfer may take place from the hottest cylinders towards the coldest: it is thus possible for one or more cylinders to fill during use. After a cylinder containing 100% liquid NH3 is closed, a slight temperature rise could suffice to exceed its allowable pressure and cause an explosion. Following these two incidents, user instructions concerning the NH3 installation were revised in order to specify the service start-up/shutdown steps, operating rules and cylinder changing procedure.