Fly ash deposition on heat-exchanger surfaces during fluidized-bed combustion of biomass causes operational problems such as reduced heat transfer and corrosion of superheater tubes. Ash deposition and alkali vapor condensation were studied during circulating fluidized-bed combustion of forest residue in a 35 MW co-generation plant. A 70 +/- 10% amount of fly ash was deposited on the heat-exchanger surfaces in the convective back pass between soot-blowing periods. Practically all the largest ash particles (d > 10 mu m) but only a small fraction of particles d < 3 mu m were deposited. The deposition efficiency of particles with a given size was not correlated with their elemental composition. About 50% of alkali chloride vapors (KCl and NaCl) were condensed in the convective back pass on fine-mode particles (d < 0.6 mu m) and the other 50% on the coarse-mode particles. Alkali chlorides were not effectively deposited since they were depleted in the largest (d > 10 mu m) ash particles.