An exothermic reaction occurred at 5.30 pm in the detoxification station of a surface treatment plant after the mixing of 300 l of rinsing water with spent acid baths. During a production change at the beginning of the afternoon, a treatment tank containing a sodium chlorite solution was drained and rinsed. According to the procedure, the laboratory manager had analysed a sample of this rinsing water, then in view of the results (low sodium chlorite content and pH = 3) had stored it in a 5,000 l tank containing 1,600 l of spent acid baths. The tank remained this way for 30 minutes, then an additional 300 l of spent acid were poured in. An explosion blew off the lid of the storage tank 2 hours later, releasing a cloud of chlorine dioxide. The laboratory manager, a laboratory technician and 2 technicians affected by the fumes were hospitalised, and another employee suffered an allergic skin reaction. Fire-fighters and a chemical emergency squad neutralised the reaction by pouring a soda solution into the tank. The site resumed its activity at 11 pm. According to the operator, the acid-sodium chlorite mixture had caused the formation and then explosion of chlorine dioxide. The low pH of the rinsing water (failure to control the process or incorrect pH measurement?) had caused the aqueous effluent to be directed to the acid bath tank. Several measures are being considered: specialisation of a treatment tank, stricter spent bath emptying and analysis procedures, improvement of product non-compatibility sheets, and specific, in-depth staff training, etc.