Experimental studies were performed to investigate the formation mechanisms of submicrometer particles during combustion of a pulverized Ohio bituminous coal. Elemental compositions of total and submicrometer ash and particle size distributions were measured at different operating conditions. The submicrometer ash particles were formed by means of a vaporization-nueleation-condensation mechanism. A decreasing collection efficiency in a cylindrical. electrostatic precipitator was observed for ash particles smaller than 100 nm due to insufficient charging. A vapor-phase sorbent injection methodology was used in the coal combustion system, and was shown to suppress the nucleation mode of the ash particles.