The characteristics of particulate matter (PM) emitted from residual fuel oil combustion in two types of combustion equipment were compared. A small commercial 732 kW rated fire-tube boiler yielded a weakly bimodal particulate size distribution (PSD) with over 99% of the mass contained in a broad coarse mode and only a small fraction of the mass in an accumulation mode consistent with ash vaporization. Bulk samples collected and classified by a cyclone indicate that 30% to 40% of the total particulate emissions were less than 2.5mum aerodynamic diameter (PM2.5). The coarse mode PM was rich in char, indicating relatively poor carbon burnout, although calculated combustion efficiencies exceeded 99%. This characteristic behavior is typical of small fire-tube boilers. Larger, utility-scale units firing residual oil were simulated using an 82 kW rated laboratory-scale refractory-lined combustor. Particulate matter emissions from this unit were in good agreement with published data including published emission factors. These data indicated that the refractory-lined combustor produced less total but more fine particulate emissions, as evident from a single unimodal PSD centered at.