The addition of sewage sludge to coal and the manipulation of the near burner aerodynamics in a 0.5 MWth pulverised fuel combustion facility were undertaken to examine their influence on the composition of fly ash. Tests revealed that the concentrations of nine metals in fine fly ash were reduced for a burner in which the fuel and air mixing was slower, producing a long flame with a more diffuse time-temperature history. The weaker bonding of the metal species to the organic matrix of sewage sludge causes increased enrichment in the finest particles. A mathematical model has been deployed capable of predicting the toxic metal emissions and their partitioning between different size fractions. The predictions indicate that the concentrations of metal vapour species, generated in the hot flame zone, are sufficiently low so that nucleation of metal species does not take place. Instead, these metal species become enriched in fly ash particles through the condensation from the metal vapour phase to form layers on the particle surfaces. Predicted concentrations in the smallest ash particles for two specific metals with known deleterious environmental impact, namely Cd and Pb, are shown to be in general good agreement with experimental data. The discrepancies are attributed to the effect of surface chemical reactions, which, in the absence of kinetic data, are not modelled.