The U.S. Bureau of Mines constructed a facility to study the self- heating of a large coal mass under conditions that simulate a gob area of a mine. The coal that accumulates in these areas and the degree of ventilation can create optimum conditions for self-heating to occur. The facility consists of a 1.8- By 1.8- By 4.6-M-long, insulated enclosure that can hold 14 tons of coal, a forced ventilation system, and computer-controlled temperature and gas measurement systems to monitor the heat and mass transfer processes that occur in the coalbed. Three tests were completed using high- volatile c bituminous coals that exhibited high spontaneous combustion potentials in laboratory-scale tests. In the first two tests, sustained self-heating was not achieved, with maximum temperature increases of 6 deg and 9 deg c, respectively. In the third test, temperatures throughout the coalbed increased steadily from the start of the test, with thermal runaway occurring near the center of the bed after 23 days. The reaction zone then moved towards the front of the coalbed, seeking oxygen, where coal temperatures reached 340 deg c, 1 ft into the coalbed. The results of these tests showed that the self-heating of a large coal mass depends not just on the rank of the coal, but critically on the particle size of the coal, the freshness of the coal surfaces, the heat of wetting, and the availability of o2 at optimum ventilation rates.