Abstract: At 3:00 a.m. on June 11, 1999, Reactor #1, part of the ABS polymerization process at Dow Chemical Company, began to overheat as the viscosity increased and threatened to stop agitation. This would cause a runaway reaction and ultimately result in an explosion. A small leak had developed in the lower bushing of the agitator and the employer instructed an employee to tighten it with a wrench. The employer replaced the normal feed (a mixture of styrene monomer, ground rubber, and acrylonitrile) with pure styrene monomer, which has a much lower viscosity, to "flush" the process in the hope that this would stop the leak. The mixture began to spill through the lower agitator packing and at approximately 2:30 p.m., there occurred a major spill of styrene monomer (flammable) and acrylonitrile (flammable and carcinogenic). Dow evacuated the plant and called for outside assistance to stop the spill and initiate clean-up. The clean-up was completed on June 12, 1999. This was the third near-miss that Dow experienced on this unit between August, 1998 and June 11, 1999. No information on employee injuries was provided.