The event occurred at the chlorine drying towers. The towers, made of PVC, exploded suddenly and violently in a mercury amalgam cell chlorine plant. Because of failure of the electrical power system a circuit breaker tripped out the mercury pumps stopped and the steel bottom plates in the cells became exposed. The alarm did not work since both the AC power supply to the mercury pumps and that for the alarm system were taken from the same source. Meanwhile the DC power and brine supply to the cells were not interrupted. Therefore the hydrogen generated at the steel plate cathode and oxygen at the anode at a mole ratio of 4:1, forming an explosive gas mixture passed to the chlorine drying towers. Concentrated sulfuric acid is used to irrigate the towers for drying of the wet chlorine gas. The most likely source of ignition in the towers was the discharge spark from an electrostatic charge caused by sulfuric acid drip. The actual electrostatic charge of a PVC-made chlorine drying tower was then measured. A static potential of minus 5 kV was constantly detected near the hole which was drilled in the side wall of the space below the Raschig ring layer. The main conclusion, obtained from the studies, is that the towers should be constructed of an acid-proof material in the future, so that the towers can be held at the earth potential together with sulfuric acid to prevent electrostatic charges building up. Three towers were completely destroyed except for the bottom structures which were made of steel. Window glass in the plant was broken within 50 m. One operator was killed and twelve seriously injured. They were all engaged in the repair work on the second tower.