Ash deposition on heat-exchanger surfaces in boiler systems can cause numerous problems, including slagging, fouling, and corrosion. These deleterious processes can be compounded if the boiler combustion process is changed from air to oxy-fuel. In this paper, fly ash deposition characteristics under both air and oxy-fuel combustion conditions were investigated using a bench-scale fluidized-bed combustor (FBC) based on measurements of ash deposition rates via a temperature-controlled probe. Three different combustion atmospheres were studied, and results demonstrated that, under similar combustion temperature profiles and equivalent fluidization velocities, the deposition rate increased when transitioning from combustion atmospheres consisting of 21% O-2/79% CO2 to air to 30% O-2/70% CO2. To determine the primary factors associated with the observed variations in deposition rates, the chemical compositions and micromorphologies of ash and fly ash deposits were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES) and scanning electron microscopy (SEM). Particulate matter with aerodynamic diameters less than 10 mu m (PM10) was measured by an electrical low-pressure impactor (ELPI), and the particle size distributions (PSDs) and carbon contents of the collected filter ash were also ascertained. The results indicate that the higher deposition propensity associated with a 30% O-2/70% CO2 atmosphere can be largely attributed to a wider PSD rather than any changes in the chemical compositions of the fly ash or deposited ash, in which there are no obvious differences between air and oxy-fuel combustion. In addition, the slightly higher concentration of fine particles produced under this atmosphere also promotes the deposition of fly ash.