Co-combustion of sewage sludge with coal or wood has been investigated in the 12-MWth circulating fluidized bed (CFB) boiler at Chalmers Technical University. The investigation focuses on emissions of trace metals from co-combustion compared to mono-combustion in CFB. The results show that co-combustion can be carried out in a CFB plant designed for the base fuel without exceeding EU emission limits for practically interesting sludge energy fractions. This was also the case for the mercury emission. In general, the ashes are enriched by trace elements with increasing share of sludge, especially in the case of wood when the ashes are dominated by the sludge properties also at small fractions of sludge addition. In the present tests the trace metals were trapped in the fly ashes regardless of base fuel. (C) 2004 Elsevier Ltd. All rights reserved.