Within the scope of the European Demonstration project RECOFUEL the behaviour and fate of chlorine and mercury during co-combustion of Solid Recovered Fuel (SRF) was investigated in detail, focusing on the environmental compatibility, and based on a lab-scale approach. In this context the term co-combustion describes the thermal utilisation of pre-treated waste-derived fuels such as SRF in industrial furnaces or power plants as a supplemental fuel. Following the definitions of the European initiative on SRF standardisation CEN/TC 343, these kind of fuels have to be produced from non-hazardous bio-residues, mixed- and mono waste streams. It is one of the key aspects of the production process to reduce the content of waste-borne pollutants such as chlorine or heavy metals. Nonetheless, the efficiency of the production process is limited and the residual pollutant concentrations may exceed those of coal to a certain extent. The experiments, focusing both on chlorine and mercury, indicated that the co-combustion process increases the flue gas concentration of gaseous chlorine species, primarily hydrogen chloride, as a result of waste-borne chlorine. Furthermore, significant differences were observed for ash-bound chlorine, which cannot be explained with the increased chlorine concentration in the gas phase alone. Considering the trace element mercury, increased chlorine inputs did not necessarily lead to increased ionic mercury shares. Furthermore, no or only insignificant amounts of gaseous elemental mercury Hg-0(g) could be determined downstream the fly ash filter system, while residual HgCl2(g) concentrations do not differ significantly. (c) 2006 Elsevier Ltd. All rights reserved.