SynopsisPoly(ethy1ene terephthalate) containing hexabromobenzene, tricresyl phosphate, or a combination of triphenyl phosphate and hexabromobenzene, pentabromotoluene, or octabromobiphenyl was extruded or spun at 280°C into monofilaments or lowdenier yarn, respectively. Only combinations of the phosphorus-and halogen-containing compounds resulted in flame-retardant poly(ethy1ene terephthalate) systems, without depreciating their degree of luster and color quality. The melting temperature, the reduced Viscosity, and the thermal stability above 400°C of these flame-retardant systems were in most cases comparable to those of poly(ethy1ene terephthalate) itself. Phosphorus-bromine synergism was proposed with flame inhibition occurring mostly in the gas phase.