In this study cyclone fly ash from a grate-fired combustion plant using forest residues as fuel was split into particle size classes with mass median diameters of 2.0 mu m, 43 mu m, 9.1 mu m, 18 mu m and 43 mu m by use of an air classifier. The particle classes were analysed for total carbon, Na, K, Mg, Ca, Sr, Ba, Ti, V, Cr, Mo, Mn, Fe, Si, Co, Ni, Cu, Zn, Cd, Hg, B, Al, Pb, As, Sb, Bi, Cl-, NO3-, Po-4(3-) and SO42- and the size dependence of the concentrations c was modelled using the relation c similar to 1/d(N). For several components a considerable dependence of the concentration on the particle size was observed. Increasing concentration with decreasing particle size was found for K, Bi, Cd, Cu, Hg, Pb, Zn and for Cl-, NO3- and SO42-. The metals Al, As, Ba, Fe, Si, Ti and V showed the opposite behaviour. Other components, including TC, Ca, Mg, Po-4(3-), Na and Mn showed no size-dependence. Investigation of single particles by scanning electron microscope in combination with energy-dispersive X-ray spectroscopy revealed that the analysed concentrations are representative for the particle classes only, the composition of single particles in a partide class can deviate considerably. (C) 2014 Elsevier B.V. All rights reserved.