Many commercially available breathing-air devices are made with materials known to ignite and burn in environments containing both oxygen and nitrogen, some with less than 19 percent oxygen. A greater hazard exists in devices that supply oxygen-enriched breathing air, sometimes at high pressures. NIOSH currently certifies breathing-air devices through functionality tests. NIOSH does not control the use of materials in compressed air and other oxygen-enriched gas systems. A program to evaluate the compatibility of materials in breathing-air devices was initiated. Commonly used materials such as Viton, Teflon, and stainless steel were selected and subjected to tests such as mechanical impact, promoted combustion, and autogenous ignition temperature. Results indicate that the combined effect of pressure and oxygen concentration on the flammability properties is complex. The results of this program will be used to help evaluate designs of devices submitted for certification.