The main fine particle (d(p) < 1 mu m) forming elements found in combustion gases of anthropogenic waste or biomass fired boilers are typically K, Na and Cl, possibly complemented with S. When these are excluded, in solid recovered fuel (SRF) combustion the main elements were found to be: Ba, Br, Cr, Cu, Fe, Pb, Sb,Sn and Zn. Fine particle composition is presented for 6 different furnace heights of a bubbling fluidized bed (BFB) boiler. As the fine particles are formed of vaporized ash species the experimental results are discussed with the support of thermodynamic equilibrium modeling for estimating the forms of the gaseous elements in the furnace. The occurrence of bromine was found to be similar to chlorine as the main forms of bromine in the furnace were estimated to be KBr(g) and NaBr(g) complemented with CuBr3(g). It is proposed that the trace elements mentioned originate mainly from plastics and rubber where they are used as production additives, stabilisers, dyes, colorants and flame retardants. Cr, Cu and Zn may originate to a large extent from alloys and other metallic impurities. SEM-EDS analyses carried out for the SRF supports the postulated origin of the elements. (C) 2011 Elsevier B.V. All rights reserved.