By installing a custom designed air-cooled probe in the freeboard region of a bubbling fluidized-bed combustor, the ash deposition on a heat transfer surface was investigated under different conditions including fuel types (lignite/peat), fuel blending ratios (20% to 80% on a thermal basis), air to fuel (A/F) ratios (1.4/1.6) and feedstock particle size. Co-firing lignite and peat at various blending ratios led to significant differences in the concentrations of some major inorganic compounds (e.g. SiO2, CaO, MgO, SO3, etc.) in the deposited ashes. Combustion of peat pellets alone caused higher ash deposition than combustion of lignite alone because of the larger contents of alkali metals, alkaline earth metals, and halogen elements (Cl and Br) combined with smaller SiO2and Al 2O3 contents in the peat. A higher A/F ratio retains lower boiling point ash elements (such as alkali metals and chlorine) in the bottom ash, leading to reduced relative ash deposition rates and retarded chloride condensation in the combustion process. Combustion of a fuel with finer particle size reduces the ash deposition rate and decreases the condensation of corrosive alkali chlorides on the heat transfer surface.