Autoignition temperature and mechanical impact tests were conducted on three rigid plastics and five elastomers in various oxygen concentrations. For Nitrile, Viton®, and Zytel®, increasing oxygen concentrations resulted in decreasing autoignition temperatures (AIT). The effect of oxygen concentration on the AIT was minimal for PTFE, Kel-F® 81, and Silicone. For the entire range of pressures evaluated (3.4 to 10.3 MPa), increasing pressures resulted in increased AIT's for the halogenated rigid plastics and for Nitrile Rubber. The pressure had a negligible effect on the AIT of Viton and Silicone Rubber for the entire range of oxygen concentrations. Materials ranking in decreasing order of autoignition temperatures remained unchanged within the oxygen concentrations and total pressures used. For all materials, increasing the test pressures from ambient to 20.7 MPa resulted in decreasing mechanical impact energy thresholds. With the exception of EPDM and Neoprene, this trend continued for all materials up to 34.5 MPa. The energy threshold for EPDM increased slightly with increasing pressures from 20.7 to 34.5 MPa, while for Neoprene, the energy threshold increased with increasing pressures from 20.7 to 27.6 MPa. Under most conditions, the rigid plastics had better performance than the elastomers in mechanical impact tests. Under conditions of higher impact energies, Nitrile Rubber was less reactive than Viton Under conditions of higher impact energies, Nitrile Rubber was less reactive than Viton and Silicone. The data indicates that for an adequate ignition-risk assessment, the mechanical impact results should be used in conjunction with other test results to simulate other possible operational failure conditions.