AbstractThe Diels‐Alder adduct of cis‐4‐cyclohexene‐1,2‐dicarboxylic anhydride (II) with hexachlorocyclopentadiene, 2,3‐dicarboxy‐5,8‐endomethylene‐5,6,7,8,9,9‐hexachloro‐1,‐2,3,4,4a,5,8,8a‐octahydronaphthalene anhydride (compound III), has been incorporated as the fire‐retardant component in polyester compositions. The structure of compound III possesses inherent properties which are exhibited as improved thermal and light stability in the derived polyesters; these improvements are based upon comparisons with polyesters prepared from the chlorendic anhydride, the fire‐retardant moiety in some commercial self‐extinguishing polyesters.