A mathematical model for predicting the fate of toxic heavy metals during combustion is presented. The model accounts for the formation of new particles through nucleation and the growth of existing particles through the combined effects of condensation and coagulation. A key simplification enabling the development of a usable methodology is the subdivision of the particle size distribution into just two modes, one comprising fine particles and the other coarse particles. The toxic heavy metal calculations are embodied in a post-processor appended to a parent code for the prediction of the aerodynamics and combustion. Three flames are studied for a coal fuel, sewage sludge fuel and a blend of these two fuels. The partitioning and emissions of the semi-volatile metals, lead and cadmium, an predicted and compared with data collected in a large-scale laboratory combustor. The predicted metal enrichments on the small particles are remarkably well-predicted. However, it is concluded that heterogeneous condensation of the metal vapours alone is insufficient to explain fully the fate of metal vapours. Surface reaction and gas phase chemical reaction should also be considered, but the necessary chemical kinetic data are currently lacking. The results indicated that ash mineral matter, such as kaolinite, is an effective sorbent for lead, but not for cadmium. As expected, the coagulation of fine particles is influenced more by the particles number density than by combustion variables. It is also found that the scavenging of fine particles by coarse particles has little effect on the metal partitioning. (C) 2000 Elsevier Science B.V. All rights reserved.