Abstract: Employee #1 was cleaning the inside of an underground gasoline storage tank in preparation for its conversion to diesel storage. The gasoline storage tank was not ventilated prior to entry, despite the fact that there was residual gasoline remaining in the tank. Testing of the atmosphere inside the tank was not done prior to entry. Employee #1 was lowered into the tank via a lifeline hoisting cable through the single man-way at the south side of the tank. Employee #1 disconnected from the lifeline when he reached the floor of the tank and walked to the north end of tank to begin cleaning. Unhooking from the lifeline/tripod upon entering the space invalidated the non-entry rescue mechanism and procedure. Employee #1 wore a full-face air-purifying respirator to enter the tank. A fully charged Self Contained Breathing Apparatus was available, but was not used. Employee #1 was overcome by the unsafe atmosphere within the tank (oxygen deficiency) and was seen staggering and attempting to reattach to the lifeline by attendants observing from above. Employee #1 was unable to reattach the lifeline, fell to the ground and lay there unconscious. Employee #1 was retrieved from the tank but died on the way to the hospital.