A driver connected one of the three transported containers to a tank containing 400 litres of 35% sodium bisulphite solution. A technician observed bubbling of the liquid in the tank and informed the driver who promptly stopped the transfer. A 98% sulphuric acid container that was properly labelled but covered by a plastic cover was accidentally connected instead of the bisulphite tank. The 5 to 10 litres of transferred acid reacted with the bisulphite. An SO2/SO3 cloud entered a neighbouring building effecting eight people located between 15 and 30 m from the unit. The eight people took ill and were hospitalised (3 for over 24 hours) and 150 employees were evacuated for 4 hours. The tank was inspected. The isolation and ventilation of the premises were improved and safety guidelines made more stringent.