Fluidized-bed combustion has been increasingly applied for combustion of low-grade fuels, such as solid biomass and waste. Sometimes the use of certain fuels may be limited due to unanticipated deposition and corrosion in the boiler. Consequently, mechanistic understanding of the behavior of ash-forming compounds in fluidized-bed combustion is crucial for further increase in the use of fluidized-bed combustion for biomass and waste fuels. The mechanisms of ash formation during circulating fluidized-bed combustion of two wood-based biomass fuels, forest residue and willow, were determined experimentally at a 35 MW cogeneration plant. In-duct fly ash samples were collected in two locations in the boiler. The fly ash particle mass size distributions were determined with a low-pressure impactor. In addition, samples of fly ash from electrostatic precipitator (ESP) hoppers, bottom ash, sand, and fuels were collected periodically for analysis. Flue gas composition and process parameters were monitored throughout the experiments. Approximately 25% of the ash was removed from the furnace as bottom ash. The bottom ash was found to be formed by deposition of the ash particles on the surface of the quartz sand and by diffusion of the ash compounds into the sand. Fly ash consisted of two distinctly different modes. Fine fly ash particle mode was formed by nucleation of volatilized species and contained mainly KCl and K 2 SO 4 during combustion of forest residue and willow, respectively. Coarse fly ash mode contained particles which were irregular agglomerates, and they were formed from the non-volatile ash species by coalescence and agglomeration inside the char particles and on their surfaces. The agglomerate structure of the coarse ash was effective in capturing volatile species in coarse particles, and it may have a significant effect on the deposition tendency of the particles.