The performance of retimet as a flame arrester on explosion-proof electrical enclosures was investigated by the U.S. Bureau of Mines. Four grades of retimet, a stainless steel foam, were evaluated: 45 nc-13, 45 nc-7, 80 nc-13, and 80 nc-7. Explosive mixtures were prepared by a dynamic flow system. A series of 30 tests were conducted on each grade of retimet in a 9.7 pct methane-air mixture. Ignition was by a low-voltage arc. Each grade of retimet successfully arrested the flame front in all methane-air tests. To establish a safety factor, a series of 30 tests were conducted in various mixtures of methane, hydrogen, and air on all four grades. A safety factor was defined as s.F.=V2/v1, where v2 is the flame front velocity in methane-hydrogen-air and v1 is the flame front velocity in methane-air. The maximum hydrogen concentration of methane-hydrogen-air in which each grade of retimet could safely function was then determined to be 10% h2-6.33% Ch4 for grade 45 nc- 7, 15% h2-4.75% Ch4 for grade 45 nc-13, 20% h2-3.17% Ch, for grade 80 nc-7, and 25% h2-1.58% Ch4 for grade 80 nc-13.