Brominated aromatic compounds like brominated benzenes, diphenylethers and dibenzodioxins adsorbed on the surface of fly ash from a municipal waste incininerator give mixed brominated/chlorinated und completely chlorinated aromatic compounds. These consecutive halogenation-dehalogenation reactions proceed by a nucleophilic mechanism, which is favoured by a high concentration of chloride on the fly ash. Results of kinetic and stereoselective behavior of these reactions will be discussed. The relevance of these results for PCDD/PCDF formation from bromine precursors in municipal waste incinerators will be discussed.