Addition of red phosphorus in concentrations of about 4% to poly(ethylene terephthalate) (PET) reduces the flammability of that polymer. The rates of flame propagation and the ignitability are reduced, while the oxygen index (O.I.) is increased. The surface temperature of burning PET amounts to TS ≈ 380°C; addition of 4% red phosphorus raises this value to TS ≈ 450°C. An increase of the environmental temperature TE enhances the flammability of PET and PET + phosphorus samples; the O.I. decreases and the rate of flame propagation increases with temperature. The flame-retardant effectiveness of red phosphorus is reduced if the sample is burned in a N2O atmosphere. This indicates that part of the flame retardancy imparted by phosphorus involves gas-phase inhibition. The major flame-retardant action does, however, occur in the condensed phase, since the rate of pyrolysis of PET is affected by the presence of red phosphorus.