The characteristics of the submicrometer mode of ash and mercury speciation on combustion of coal in an oxygen-carbon dioxide and air (conventional) system were compared. The experiments were conducted at different O-2:CO2 and O-2:N-2:CO2 mixing ratios. On replacing the nitrogen in air with carbon dioxide, the total mass of the particles in the submicrometer mode (< 0.5 mu m) of ash was smaller. Correspondingly, the geometric mean size (d(pg)) of the submicrometer aerosol was smaller by approximately 28%. When the O-2:CO2 ratio was increased from 1:4 to 4:4, the geometric mean size of the submicrometer mode increased from 29 to 54 nm because of a faster vaporization rate as a result of a higher coal particle temperature. An increase in the geometric mean size was observed on increasing the N-2:CO2 ratio at a fixed O-2 concentration because of the same reasons. The shape of the primary particles was spherical in all the tests, indicating that a vapor to particle transformation pathway was prevalent. The ratios of elemental to oxidized mercury (approximately 4: 1) were similar for O-2-CO2 and air combustion.