Abstract: This specification discloses that certain polychlorinated or polybrominated aromatic hydrocarbon compounds can be incorporated in thermosetting polymer compositions to make the thermosetting compositions flame retardant without injuring the desirable physical and electrical properties of the thermosetting compositions when in the thermoset state. These polyhalogenated aromatic hydrocrbons are apparently caused to react into the thermosetting composition, provided there is a minimum of 1 mol of a polyunsaturated monomer containing carbon to carbon unsaturation for each 3 mols of polychlorinated aromatic hydrocarbons.