AbstractA study has been made of the effect of antimony and molybdenum oxides on the temperature‐critical oxygen index relationship of halogenated polyester thermosets. The polyesters were prepared from maleic anhydride, phthalic anhydride and propylene glycol, using either tetrachlorophthalic anhydride, tetrabromophthalic anhydride or chlorendic anhydride as the halogen source. Marked differences were observed in the effect of the two oxides. The critical oxygen index values of the halogenated polyesters containing maoybdenum (VI)oxide decreased with increase in timperature, which is in accordance with what is normally found for organic polymers, with and without additives. In contrast to this, the critical oxygen index values of the formulations containing antimony(II)oxide increased with increased with increase in temperature to reach a maximum value at around 100–150 °C. These results indicate that antimony(II)oxide is a better flame retardant than molybdenum(VI)oxide, since not only is it superior at the primary ignition stage but it shows enhanced activity at the higher temperatures which would be experienced by polymeric material in the early stages of a fire. It is concluded that higher temperature critical oxygen index studies provide a better guide to the flame retardant activity of additives than do ambient critical oxygen index determinations a