A process for brominating a styrenic polymer in an organic solvent in the presence of an antimony trihalide catalyst such that a reaction mass containing brominated styrenic polymer in an organic phase is formed is improved by mixing hydrochloric acid or hydrobromic acid, or both, with said reaction mass at least once to extract antimony catalyst residues from said reaction mass as an acidic aqueous phase. Preferably, catalyst residues are recovered from the acidic aqueous phase and still more preferably, recovered antimony trihalide catalyst residues are recycled to constitute at least a portion of the antimony trihalide catalyst used in preparing the brominated styrenic polymer.