Organic materials are made flame retardant by addition of a polyhalogenated polyphenylene sulfide such as decabromodiphenyl sulfide or 1,4-thiobis(2,4,6-tribromophenyl)-2,3,5,6-tetrabromobenzene alone or in combination with a Group VA metal oxide, e.g. Sb.sub.2 O.sub.3. A process for making the compounds is also disclosed.