Polyhalogenated dibenzofuranes having at least six halogen atoms are prepared by halogenation of 2,4,8-trichlorodibenzofurane or of dibenzofurane. The halogenation may be achieved by Cl.sub.2, Br.sub.2, SO.sub.2 Cl.sub.2, S.sub.2 Cl.sub.2 or similar halogen donors. The products are flameproofing agents for polymers, especially for polyesters and polyolefins. Examples are octachloro-, trichlorotribromo- and trichlorotetrabromo-benzofurane.