In a pharmaceutical product manufacturing plant, an explosion occurred when a hydrogenation reactor was commissioned for the first time during joint tightness tests conducted in an atmosphere of hydrogen at very high pressure. The accident resulted due to the rupture of one of the joints tested followed by the auto ignition of a 30 litre air/H2 mixture. Five employees near the reactor who sustained burns and suffered from ear pain due to the overpressure were hospitalised. Among them, 2 employees were hospitalised for over 48 hours (humming sound in the ears). Material damage was confined to the immediate perimeter of the reactor. The H2 detectors controlling the closing of the H2 supply valve were not yet installed but this was not critical (prompt reaction taken by employees). The preliminary tests carried out under nitrogen were insufficient.