The ASTM oxygen index, a standard atmospheric pressure test for comparing the flammability of plastics, has been used in combination with other tests to evaluate the suitability of nonmetals for use in oxygen service. In 1983, this test was extended to allow measurements at pressures to 2 MPa (300 psig). This paper describes how the concept of a pressurized oxygen index test can be extended further to measure the flammability of metals and hence their suitability for use in oxygen service. An experimental program is described that measured oxygen indexes for carbon steel pipe (25 mm [1.0 in.] diameter by 4.8 mm [0.188 in.] wall) as a function of pressure to 20 MPa (3000 psig) at temperatures of about 300 and 355 K (80 and 180°F). Oxygen/nitrogen mixture velocities inside the pipe varied from 0.15 to 0.90 m/s (0.5 to 3 ft/s). An experimental system was developed, including the test specimen configuration and a thermite ignition pill for use in oxygen mixtures. Measured oxygen indexes were 81 mol% oxygen in nitrogen at 1.03 MPa (150 psi), 53 mol% at 6.9 MPa (1000 psi), and 51 mol% at 20.7 MPa (3000 psi). Additional tests demonstrated the effectiveness of Monel as a firebreak material and Buna N rubber as an ignition source for carbon steel.