Combustion aerosol measurement methods were introduced and applied for extensive ash formation studies at four operating recovery boilers in Finland. Ash particle mass size distributions determined with a Berner-type low-pressure impactor downstream the heat exchangers were clearly bimodal with the fine mode at about 2 mu m and the coarse mode above 3 mu m aerodynamic diameter. According to SEM images, fine ash mode consists of individual, almost spherical 0.3-0.7 mu m alkali salt particles and of agglomerates with few primary particles of similar diameter and shape. The degree of fine mode primary particle sintering increased when increasing boiler heat load. Coarse mode includes large agglomerates with up to thousands of 0.3-0.7 mu m alkali salt primary particles and spherical silica particles. Ash particle main component was sodium sulfate as determined with X-ray diffraction. Sodium-to-sulfur molar ratio of ash particles calculated on the analyses results with an ion chromatography decreased from the upper furnace sampling point to electrostatic precipitator inlet conditions, indicating sulfation of ash particles within the heat exchanger section. Chlorine in ash was bound as sodium chloride, no potassium chloride was detected with X-ray absorption fine structure spectroscopy. Furnace measurements showed that fine mode ash particles are formed already in the furnace via vapor condensation. The extents of release of 12% for Na, 24% for S, and 48% for Cl were determined on the basis of ion concentrations in fine particles and the mass balance calculation on the recovery boiler. Coarse particles observed downstream the heat exchangers are proposed to form mainly via entrainment of large agglomerates of fine ash particles deposited on the heat exchangers. The fine mode particle size was insensitive to the furnace conditions although the particle concentration increased when the furnace gas temperature increased. This and the increase of Na/S molar ratio in the particles indicates that Na volatilization increases with the increasing furnace temperature, whereas S release is less sensitive to the temperature increase.