Flame-retardant additives used in plastics are based on halogen, phosphorus, inorganic compounds, and minerals. In this study, the use of bismuth trioxide as synergistic agent with brominated flame retardant is investigated and compared to formulations with antimony trioxide. Decabromodiphenyl ether and metal trioxide were incorporated in a polymer matrix via single-screw extrusion. The samples were classified with the UL 94 vertical burning protocol and studied with thermogravimetric analysis. The general combustion and heat release behaviors were studied via cone calorimeter. Results showed that 4% (wt) Bi₂O₃ displayed a synergistic effect with 12% (wt) of the halogen compound to achieve V-0.