To design more effective capture technologies, it is desirable to have higher concentrations of carbon dioxide (conventional systems have a CO 2 concentration of approximately 13%) in coal combustor exhaust gas streams. To overcome this issue of low CO2 concentrations in coal combustion exhausts, it has been proposed that combustion of coal in CO 2/O2 mixtures may allow the enrichment of carbon dioxide to about 95%. This is achieved by the use of enriched oxygen (eliminating the nitrogen diluent in air, thus reducing NOX emissions) in the feed stream, and recycling the exhaust gases- resulting in an enrichment of carbon dioxide. While the emission of certain pollutants such as NOX is eliminated, and the capture of CO2 potentially enhanced, the effects of this combustion system on other pollutants, such as the fine fraction of particulate matter (PM), have not yet been explored. Since the submicrometer ash particles are formed by means of vaporization, nucleation, condensation and coagulation mechanisms, their characteristics could be altered under carbon dioxide-oxygen combustion conditions, and may affect the removal efficiency in particulate control devices. An experimental study was performed to map out the variations in the particle size distribution under a variety of combustion conditions: conventional (with air), and different CO2/O2 ratios. Real time aerosol instruments were used to measure the resultant size distributions of the submicrometer mode at the exhaust of a laboratory scale combustor burning Powder River Basin (PRB) coal seams. Distinct differences were observed, and attributed due to altered combustion conditions in the presence of higher carbon dioxide concentrations. While clear differences were observed in the particle size distributions, no differences were observed in the mercury speciation.