The oxygen (oxidant O2) index of flammability is accepted as a useful, easily reproducible, and inexpensive method of assigning relative flammability ratings to various materials. There have been reports of the ignition of plastic and rubber endotracheal tubes during anesthesia for surgery when an ignition source is in close proximity. Ignition sources include lasers for laryngeal surgery and pharyngeal electrocautery. Such procedures are often performed in oxygen- (O2) and nitrous-oxide- (N2O) enriched atmospheres. This study determines and compares the oxidant O2 index of flammability for three materials commonly used in endotracheal tubes: polyvinylchloride, red rubber, and silicone. Since nitrous oxide also supports combustion, we introduce the oxidant nitrous oxide (N2O) index of flammability for the same three materials. Indices were determined using a technique similar to the one previously described for the oxygen index of flammability. The oxidant O2 index of flammability for polyvinylchloride endotracheal tubes is 0.263, for silicone endotracheal tubes, 0.189, and for red rubber endotracheal tubes, 0.176. The oxidant N2O index of flammability for polyvinylchloride endotracheal tubes is 0.456, for silicone endotracheal tubes, 0.414, and for red rubber endotracheal tubes, 0.374. The indices are linearly additive when O2 and N2O are combined. This study suggests the possibility of developing other indices of flammability using other oxidants and predicting the effect of combining oxidants.