Abstract: Employee #1 was assisting Employee #2, who was welding a pipe and valve connection onto a tank that was mounted onto a 1988 International truck, model 1954. The 3,000 gallon tank had been installed on the truck by PBS Equipment, Inc. The tank truck would be used to haul waste oil. At the time of the accident it had not yet been used. The pipe and valve connection always had to be welded onto new tank trucks. Both employees were on top of the tank welding on the front part. The tank consisted of three separate compartments. The day before the accident, about 200 gallons of oily waste water, probably containing some gasoline, had been stored inside the tank's rear compartment. No other liquids were in the tank. After the welding was completed, Employee #2 started to step from the tank to the cab and saw Employee #1 at the rear of the tank doing something, possibly lighting a cigarette. The tank exploded, hurling Employee #1 several feet. He sustained multiple traumatic injuries from which he died. Employee #2 was also thrown by the explosion and rendered unconscious. Welding or lighting the cigarette may have been the ignition source.