Abstract: A black substance accumulates in the pipelines of the injection wells that are used in an oil field water-flood operation and occasionally has to be removed. Hydrochloric acid (HCl) is pumped through the pipelines to dissolve the black matter. On June 30, 1994, Employee #1, the employer, pumped HCl through a pipeline that was 3,500 to 4,000 feet long. The acid was taken to the location in one tank truck and another tank truck was positioned to receive the acid. As the acid passed through the pipeline, it reacted with the calcium sulfide and/or iron sulfide in the black matter and created hydrogen sulfide (H2S). After the acid had been pumped through the pipeline and into the tank truck, the pump on the receiving tank truck was used to suck any remaining fluid from the hose. The pump evacuated H2S from the tank and out of the tank vent. The H2S created a lethal concentration outside the vent. Employee #1, the truck driver, walked into the vapor and immediately lost consciousness and died. Employee #2 attempted to assist his boss and was overcome by the toxic gas but recovered.