NASA White Sands Test Facility recently investigated the effects of diluents on flammability of nonmetals. Downward-flame propagation flammability tests were conducted on Teflon PTFE, Kel-F 81, and Viton samples, using helium, nitrogen, argon, and neon diluents.The tests were conducted at 6.9 MPa (1000 psia), 20.7 MPa (3000 psia), and 34.5 MPa (5000 psia) using various diluent/oxygen mixtures. The test objective was to determine the minimum diluent concentration required to extinguish the burning sample. The data indicated that at pressures of 6.9 MPa (1000 psia) and higher, the materials tested (which are among the most commonly used nonmetals in oxygen systems) would burn even in low-oxygen concentrations. Large quantities of diluents, approximately 60 percent and higher, were required to prevent sustained combustion. Helium was the most effective diluent for preventing flame propagation, followed by nitrogen, neon, and argon. The effectiveness of helium could be explained by its high thermal conductivity, which could have resulted in reduced heat-feedback to the burning material. For a given pressure increase, the increase in diluent concentration required for sample extinguishment was higher for Teflon PTFE and Kel-F 81 than for Viton. At some pressure between 20.7 MPa (3000 psia) and 34.5 MPa (5000 psia), Teflon PTFE and Kel-F 81 became flammable in air, indicating that high-pressure air systems require careful selection of appropriate nonmetallic materials and design considerations to avoid ignition sources.