During hazard analyses of oxygen-enriched systems, the flammability of polymers is commonly assessed by reviewing standard limiting oxygen index (LOI) data available in the literature and summarized in ASTM Standard Guide for Evaluating Nonmetallic Materials for Oxygen Service (ASTM G 63). The data provided by this standard may not correlate with many real-life fire scenarios. Two of the most common issues relate to the direction of flame propagation recommended by the method and the test environment temperature. Modifications of these two parameters could render most polymers flammable, which under standard testing conditions set forth by ASTM G 63 would be considered nonflammable. Modified LOI tests were conducted on 16 polymers to determine the magnitude of these effects. The results indicate that the existing data presented in ASTM G 63 should be used with caution. Furthermore, modified oxygen index data should be included in standard guides for oxygen systems design.