Abstract: On June 5, 2006, Charco Maintenance & Welding was hired by the facility owner, Petro Operating Company, to do maintenance and repair work during a shut-down of the facility. The four-man crew was replacing some valves that were attached to a 10,000-gallon capacity surge tank. The surge tank contained saltwater that was mixed with hydrogen sulfide, carbon dioxide, and hydrocarbons. The contents of the surge tank were lowered to a level below, where the valves were attached. Employees #1 and #3 climbed the fixed ladder of the tank to one of the elevated platforms to remove one of the valves. They used a MiniCheck hydrogen sulfide detector, which was manufactured by Del Mar Scientific that changes color when hydrogen sulfide is detected. They used the detector, when the valve was initially loose from its connection, but the detector did not indicate any hydrogen sulfide. During the removal of the valve, hydrogen sulfide was released into their immediate work area. It was estimated that the airborne concentration of hydrogen sulfide was approximately 500 parts per million. Employees #1 and #3 lost consciousness from the exposure. When Employee #3 regained consciousness, he climbed down from the platform. Employee #4 climbed the ladder to help Employee #1. He removed his SCBA mask to provide clean air to Employee #1, but was almost over come, and had to redon his mask. Employee #1 was lowered to the ground, but did not regain consciousness, and later died at the scene from his exposure. Employees #3 and #4 were not hospitalized for their exposure. Employee #2 was not exposed. It was determined that the employees had not been properly trained on the lock-out/tag-out program, handling the potential exposures to air contaminants, or rescue efforts.