The behaviour of some selected trace metals (Hg, Cd, As, Pb, Sb, Cr, Co, Cu, Mn, Ni and V) during co-combustion processes of bio-waste materials (sewage sludge, waste wood, refused derived fuel) and coal has been predicted by thermodynamic equilibrium calculations using the HSC-Chemistry 4.0 software. The influence of temperature, flue gas composition, trace element concentration and minor fly ash components on equilibrium composition was evaluated. For most of the elements, an increase in the HCl concentration favours the formation of gaseous species while increasing concentration Of SO2 in the gas composition enhances the formation of condensed species. Trace element interactions with minor fly ash components were predicted. From results obtained in this study it may be concluded that, from a thermodynamical point of view, the addition of a secondary fuel in combustion processes does not produce an increase in trace element emissions to the environment. Generally, trace elements are captured in ashes avoiding that these elements reach the stack. (c) 2005 Elsevier Ltd. All rights reserved.