Abstract In many countries, legislation has been enacted to set guidelines for ambient concentrations and to limit the emission of fine particulates with an aerodynamic diameter less than 10 μm (PM 10 ) and less than 2.5 μm (PM 2.5 ). Ash particles are formed during the combustion of coal in pf boilers and fine ash particulates may potentially pass collection devices. The ash size fractions of legislative interest formed during coal combustion are the result of several ash formation mechanisms; however, the contribution of each of the mechanisms to the fine ash remains unclear. This study provides insight into the mechanisms and coal characteristics responsible for the formation of fine ash. Five well characterized Australian bituminous coals have been burned in a laminar flow drop tube furnace in two oxygen environments to determine the amount and composition of the fine ash (PM 10 , PM 2.5 and PM 1 ) formed. Coal characteristics have been identified that correlate with the formation of fine ash during coal combustion. The results indicate that coal selection based on (1) char characterization and (2) ash fusion temperature could play an important role in the minimization of the fine ash formed. The implications of these findings for coal selection for use in pf-fired boilers are discussed.