There has been increasing concern in recent years about the ignition sensitivity of polytetrafluoroethylene (PTFE) lined flexible hoses used in oxygen service. Stainless steel braided flexible hoses lined with PTFE were tested in a high-volume pneumatic-impact system. The objective of the testing was to characterize the ignition mechanism by determining the effects on ignition of impact pressure, pressurization rate, and volume upstream and downstream of the flexible hose. Ignitions occurred at impact pressures well below the working pressure of the hoses, and at pressurization rates that can easily be obtained with manually operated valves. The hoses ignited at the downstream end and combustion propagated back toward the source of fresh oxygen. The addition of stainless steel hardline downstream from the hose prevented ignitions at all pressures and pressurization rates. Internal observations revealed evidence of shock ionization of the oxygen prior to ignition. Although adiabatic compression of the oxygen is the most likely source of thermal energy for ignition, shock ionization of the oxygen may play an important role in the ignition mechanism by decreasing the activation energy necessary to kindle the reaction.