An explosion ripped through a chlorine production site during the restart of an installation. The hydraulic seal had apparently become blocked by fire extinguishing products following a fire on the hydrogen exhaust system. The blockage caused the pressure to increase in the hydrogen manifold and the passage of flammable gas into the chlorine through the diaphragms. The Cl2/H2 mixture then entered the Cl2 desiccation unit before exploding. The explosion was probably initiated by a discharge of static electricity or UV radiation. The internal contingency plan was put into action. The accident did not claim any victims although the desiccation towers were destroyed and the chlorine manifold was damaged. The increase in the chlorine’s hydrogen content had been detected by an on-line analyser 15 minutes prior to the explosion. The accident demonstrated the inappropriate hydrogen firefighting means or insufficient with regard to the detection of overpressure in the hydrogen manifold and the analysis of this gas in chlorine, as well as inappropriate operating procedures with regard to the intervention procedures in the event of excessive H2 pressure or chlorine gas polluted by flammable gas. Corrective measures were implemented: Elimination of the causes of pressure build-up in the H2 manifold, reinforcement of hydrogen overpressure detection systems and the presence of hydrogen in the chlorine, adaptation of the initiator operating procedures (H2 overpressure and chlorine polluted by hydrogen).