Contents, modes of occurrence and behaviour of Cl and Br in combustion wastes from eleven Bulgarian coal-fired thermoelectric power stations (TPS) were studied. Despite some differences, there are indicative close relationships between the contents, redistributions, modes of occurrence and behaviour of Cl and Br in fly ash (FA), bottom ash (BA) and stack emissions during coal combustion. Chlorine and Br occur in inorganic and organic combinations in FA and BA. Both elements are normally present, in decreasing order of significance, as: (1) impurities in the glass phases (dominantly for Br); (2) impurities in the various crystalline components (dominantly for Cl) such as surface-bound Ca-Na-K-Mg sulphates and phosphates; (3) organic compounds in char; (4) impurities in the fluid constituents; (5) probable discrete inorganic phases. Dominant proportions (up to 96%) of the initial Cl and Br in coal are normally emitted during combustion process in TPS. The main reasons for Cl and Br volatilisation, condensation, capture and retention during coal combustion are described. The results also indicate some possible environmental pollution of the air, water and soil with Cl and Br in the areas surrounding the large TPS. (C) 2000 Elsevier Science Ltd. All rights reserved.