The paper gives results of an examination of particle behavior and particle size distributions from the combustion of different fuel oils and emulsified fuels in three experimental combustors. Results indicate that improved carbon (C) burnout from fuel combustion, either by decreasing the temperature quench rate or by forming smaller fuel droplets through the secondary atomization characteristics of oil/water emulsions, increases the volume of the submicron particle fraction. Also, the use of oil/water emulsions can increase the submicron particle volume compared to a non-emulsified oil burned in the same combustor. In contrast to larger coarse-mode particles which are composed largely of C char and inherently bound metals and sulfur, these submicron particles appear to be composed of metal sulfates that are more water-soluble than the larger coarse-mode particles. For fuel oils, submicron particle volume varies directly with C burnout, and inversely with total particle mass. These metal sulfate-enriched submicron particles are formed by vaporization and subsequent nucleation, coagulation, and condensation mechanisms. Where normal atomization, high quench rates, or other obstacles to complete combustion exist, substantial amounts of inorganic material remain bound with the unburned C char.