Abstract: A flame resistant and self-extinguishing copolyester composition is made by opolycondensing a dicarboxylic aromatic acid or its diester, a saturated aliphatic diol, a tetrabrominated diol and a sulphonated aromatic compound, preferably in the presence of a phosphor derivative as a complexing and stabilizing agent. Preferably the acid is terephthalic acid and the saturated diol is ethylene or butylene glycol. The viscosity of the copolyester is from 0.3 to 0.6 dl/g and preferably not less than 0.4 or more preferably 0.45 dl/g and its bromine and sulphur contents are 0.1-12% and 0.05-1% by weight respectively, the phosphor, if any, not exceeding 1%. A marked synergic effect of the concurrent presence of bromine and sulphur is evidenced by combustion tests.