Thermoplastic, linear, saturated polyesters containing flameproofing agents consisting of halogen compounds and an antimony trioxide which has been prepared in an aqueous suspension of an inorganic support using an aqueous antimony(III) halide solution and adding an aqueous solution of a base to raise the pH to from 6 to 8 with precipitation of the antimony compound onto the support, followed by separation of the solids from the aqueous phase.