Combustion is a significant source of fine particulate matter (PM) emissions, and heavy oil combustion is suspected of producing particulate emissions with potentially significant toxicity. This paper is concerned with the characterization of particulate emissions from a practical boiler burning heavy fuel oils. The purpose of the study was to identify possible mechanisms that might relate both the combustion process and the fuel burned to the size segregated characterization of the fine particles formed, and consequently to their propensity to cause pulmonary injury. The data presented, therefore, help suggest specific fundamental issues which define directions for future research in this area. In this initial study, samples of PM were taken from the stack of a commercial 732 kW (2.5 x 10(6) Btu/hr) rated firetube boiler burning four different heavy fuel oils, including two grades and three sulfur contents. Submicron and supermicron particle size distributions (PSDs) were measured using an in-stack cascade impactor, a scanning mobility particle sizer (SMPS), and an insitu light scattering system. Size classified bulk samples were also collected using a high volume dilution sampler. Finally, EPA Method 5 (total particulate) and Method 60 (metal analyses) samples were extracted and analyzed. Measured PSDs showed evidence of a submicron accumulation mode between 0.07 and 0.08 mu m diameter. PM less than 2.5 mu m diameter (PM2.5) which included the accumulation mode and a significant portion of a broad coarse mode, comprised between 30 and 50% of the total PM mass emissions. Small particles less than 0.25 mu m diameter contained significant quantities of metals and sulfates, while larger particles (> 2.5 mu m) were composed primarily (70-95%) of cenospheric carbon. These and related data are interpreted in the light of possible mechanisms governing the partitioning of toxic metals from heavy oil combustion.