Although attention is focused currently on the emission of particles less than 2.5 microns (PM2.5) from stationary combustion systems, ultrafine or submicron particulate matter may prove to be an important constituent of PM2.5. Field observations and theoretical considerations lead us to conclude that vaporization and condensation of ash occurs in pulverized coal-fired power plants. Furthermore, many toxic trace elements are enriched in the vaporization mode of the fly ash because many of the inorganic elements in coal are volatile at flame temperatures. In the flame zone, the inorganic vapors homogeneously nucleate to form an ultrafine aerosol with a primary particle size of 0.01 to 0.03 µm. Post-flame, the combustion gases cool rapidly to near room temperature in a time period of 2 to 5 s. The mean particle size of this condensation aerosol grows by coagulation. Calculations for a typical vapor loading show that the peak of the mass distribution grows to 0.03 to 0.07 µm in the range of 1 to 10 s. These ultrafine ash particles are not collected as efficiently by electrostatic precipitators as are the larger ash particles. Thus, toxic metals in the vaporization mode can be preferentially emitted to the atmosphere.