Abstract: ON JULY 29, 1985, 4,200 POUNDS OF THIOUREA DIOXIDE AND 800 POUNDS OF SODIUM SULFATE WERE MIXED IN A 5,000 POUND CAPACITY STAINLESS STEEL BLENDER. A SELF-SUSTAINING OXIDATION REACTION OCCURRED, CONSUMING ALL THE MATERIAL IN THE BLENDER. LARGE AMOUNTS OF SULFUR DIOXIDE WERE RELEASED AS A DECOMPOSITION PRODUCT. THE GAS ESCAPED FROM THE VESSEL THROUGH A GASKET WHICH RUPTURED DUE TO THE PRESSURE BUILD-UP. ONE EMPLOYEE WHO WORKED IN AN ADJACENT RECEIVING AREA INHALED SULFER DIOXIDE WHILE LEAVING THE BUILDING. HE WAS TREATED FOR PULMONARY IRRITATION BY THE COMPANY PHYSICAN. HE WAS RELEASED THE SAME AFTERNOON AND RETURNED TO WORK THE NEXT MORNING. ALL THE OTHER EMPLOYEES WERE EVACUATED WITHOUT SIGNIFICANT EXPOSURE. THE CHEMICAL REACTION COULD HAVE BEEN INITIATED BY HEAT BUILD-UP ON A FAULTY COUPLING ON THE INTENSIFIER BAR, OR BY CONTAMINATION BY MOISTURE OF THE RAW MATERIALS, OR A COMBINATION OF THE TWO FACTORS.