Abstract: At approximately 10:30 p.m. on October 8, 1998, Employees #1 through #4 were working on the fifth level of an eight-level open steel girder silane hydrogenation reactor structure. A 6 in. diameter pipe between a settler and a quench condenser burst, emitting 28,000 lb of silicon tetra chloride, 6,200 lb of trichlorosilane, and 80 lb of hydrogen under 400 psi. The first two products decomposed in the presence of atmospheric moisture to produce hydrochloric acid. The vapor cloud propagation was so rapid that none of the workers had time to reach escape respirators stored on the structure. Employees #1 and #2 were killed, Employees #3 and #4 sustained injuries that required hospitalization. According to the attending physicians, their injuries were consistent with inhalation of hydrochloric acid vapors.