Abstract: A normally flammable organic polymer is rendered flame retardant by the incorporation of a compound corresponding to the formula: ##SPC1##Wherein X is chloro or bromo, Q is halo, hydrogen, or a hydrocarbyl or hydrocarbyloxy group, and Z is a divalent saturated polybromocycloalkylene group containing 5-16 carbon atoms and 2-6 bromine atoms. Such flame retardants include, e.g., 5,6-dibromo-1,10,11,12,13,13-hexachlorotricyclo[8.2.1.0.sup.2,9 ]tridecene-11, i.e., the dibrominated 1:1 Diels-Alder adduct of hexachlorocyclopentadiene and cyclooctadiene-1,5. If desired, the flame retardant may be employed in conjunction with a synergist, such as antimony trioxide.