The paper gives results of a comparison of the characterisitics of particulate matter (PM) emitted from residual fuel oil combustion in two types of combustion equipment. A small commercial 732-kW fire-tube boiler yielded a weakly biomodal particulate size distribution (PSD) with > 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 30-40% of the total PM emissions were < about 2.5 micrometers aerodynamic diameter (PM2.5). The coarse mode PM was rich in char, indicating relatively poor carbon burnout, although calculated combustion efficiencies were > 99%. This characteristic behavior is typical of this type of small boiler. Larger scale, utility units firing residual oil were simulated using an 82-kW laboratory-scale refractory-lined combustor. PM emissions from this unit were in good agreement with published data, including published emission factors. These data indicate that the refractory-lined combustor produced lower total but greater fine particulate emissions, as evident from a single unimodal PSD centered at about 0.1 micrometer diameter. Bulk cyclone-segregated samples indicated that all the PM were smaller than 2.5 micrometers aerodynamic diameter.