Abstract: On November 29, 1993, Employees #1 and #2, mechanics at Kopper's Industries, Stickney, IL, had ascended a coal tar pitch distillation unit to open a manway. The mechanics were to remove all but four bolts on each of three manways. The last four bolts were to be removed by another crew at which time air-supplied respirators were to be used. Apparently, Employees #1 and #2 decided to remove more than the usual number of bolts, causing the manway gasket seal to break open and emit hydrogen sulfide gas. The mechanics were not wearing respirators at the time nor were respirators in their possession. Both mechanics were overcome by the hydrogen sulfide gas, asphyxiating Employee #1 and rendering Employee #2 unconscious. In the rescue attempt, Employees #3 through #6, the rescuers, became ill from the gas due to lack of proper respirators and possibly from improper use of respirators. Employees #1 through #6 were taken by paramedics to the hospital: Employee #1 was dead on arrival. The remaining employees were hospitalized overnight; one employee remained for several days. The investigation revealed improper procedure by Employees #1 and #2.