Bromoacenaphthylenes and their condensates as flame-retardant reagents were synthesized by bromination of acenaphthylene using ZnCl2CF3COOH or FeCl3 as catalysts and subsequent dehydrobromination. The chief components were identified as bromoacenaphthylene monomers when ZnCl2CF3COOH were used, and as their condensates (mostly trimers) in the case of FeCl3. Their performance as flame-retardant reagents for ethylene–propylene–diene terpolymer (EPDM) was evaluated by measuring the oxygen index of finished compounds, and flammability by a vertical flammability test based on UL-94-VO. Both the monomers and the condensates demonstrated high flame-retardant effectiveness. The high efficiency was attributed to their excellent dispersity in the base polymer and their characteristic thermal decomposition behavior. In TGA, they decomposed in a very wide range of temprature (ca.200–560°C), which covers the decomposition range of EPDM. This was attributed to the existence of bromines of different thermal stabilities in one molecule.