Ash formation was studied experimentally during combustion of forest residue and willow in a 35 MW circulating fluidized-bed co-generation plant. Ash particles and vapors were sampled downstream of the process cyclone at flue gas temperatures of 810-850 degrees C. Inorganic vapors and fly ash particles were collected separately on filters. Size-classified fly ash particle samples were collected with an impactor. Elemental analysis methods and electron microscopy were used for ash characterization. During combustion of forest residue, about 30-40% of the total ash and a similar percentage of Ca and P was attached on the bed-material particles and were eventually removed from the bed along with bottom ash. K was retained with a higher efficiency (about 50%). S and Cl were volatilized in the furnace and were not retained in the bed. During combustion of forest residue, 80% of S had already reacted with species present in supermicrometer ash particles when detected after the process cyclone at 810-850 degrees C. When willow was combusted, about one-half of the sulfur remained as SO2, More than one-half of the particle-phase S was present as alkali sulfate fine particles that were almost nonexistent during forest residue combustion. Cl was present as gaseous species (KCl and presumably HCl) at 810-850 degrees C with both fuels.