In a chemical plant, 1 litre of chlorine leaked out of a container in a non-confined canopy depot housing 16 1-tonne tanks (including two being emptied) and a drawdown station in vapour phase. The Cl2 detector of the depot was triggered and fire fighters wearing self-contained breathing apparatus arrived on site. Two leaks were detected: one at the nozzle flange and the other at the body flange. The tank was cooled, put away in a shaded place and a cover was placed on the nozzles to isolate them. The container was bled and the chlorine was transferred to a production unit 30 min later. The toxic cloud remained confined to the storage area and its immediate vicinity. No effects were observed outside the plant. A technical expertise of the tank revealed that the Cl2 leak undoubtedly resulted from a stuffing box that might have been loosened by a plant technician. Some valves have a reverse pitch.