One of the most important ignition mechanisms to initiate the burning of nonmetallic materials in oxygen, and directly linked to many large oxygen-system fires, is the rapid compression of oxygen against a nonmetallic material. Rapid compression, also referred to as adiabatic compression, testing of components and systems is utilized worldwide to determine their compatibility in oxygen systems. However, limited research is available on how compression energy is transferred to nonmetallic materials and leads to ignition. By characterizing the energy available during a pressure surge and the energy required for a material to ignite for a given test pressure, “safe” and “fail” zones can be determined. To this end, the concept of a “critical enthalpy” is introduced to define at what pressure there is enough energy in oxygen (after rapid compression) to raise a nonmetal material to its ignition point. This critically important information can be utilized to inform designers of the energy available in the system that is necessary to be rejected to preclude ignition of a nonmetal by rapid compression in gaseous oxygen.