Fly ash formation and behaviour during combustion of willow (supplemented by 20 wt% of wood pellets) was studied experimentally in a 3-12 MW circulating fluidized bed boiler. Particle size distributions in a size range d(ae) < 8 mu m were obtained by a Berner low pressure impactor in the convection path at a temperature of 650 degrees C and downstream the convection path (160 degrees C). For particles above 8 mu m size analysis was performed from a cyclone sample collected at 160 degrees C. Size distributions contained two distinctive modes. The major inorganic elements in the coarse mode particles (d(ae) > 1 mu m) formed from the non-volatilised ash residue were Ca, K, P, Mg and S. The fine ash particles (d(ae) < 1 mu m) formed from vaporised ash forming constituents were composed of alkali (mainly potassium) sulphates, chlorides and, according to chemical equilibrium calculations, carbonates. K2SO4 was condensed prior to our sampling station at 650 degrees C. Condensed KCl and K2CO3 were observed only at the latter sampling station at 160 degrees C. About 50% oi the coarse ash particles were deposited on the heat exchanger surfaces between 650 degrees C and 160 degrees C, whereas the concentration of the fine mode sulphate particles was not observed to decrease from 650 degrees C to 160 degrees C.