Combustion experiments were carried out on four different residual fuel oils in a 732 kW boiler. Particulate matter (PM) emission samples were separated aerodynamically by a cyclone into fractions that were nominally less than and greater than 2.5 microns in diameter. However, examination of several of the samples by computer-controlled scanning electron microscopy (CCSEM) revealed that part of the 2.5 micron samples (PM{sub 2.5+}). Inorganic sulfides and elemental sulfur were present in lower percentages. The Ni XANES spectra from all of the samples agree fairly well with that of NiSO4, while most of the V spectra closely resemble that of vanadyl sulfate (VO{center_dot}SO{sub 4}{center_dot}xH{sub 2}O). The other metals investigated (Fe, Cu, Zn, and Pb) were also present predominantly as sulfates. Arsenic is present as an arsenate (As{sup +5}). X-ray diffraction patterns of the PM{sub 2.5} fraction exhibit sharp lines due to sulfate compounds (Zn, V, Ni, Ca, etc.) superimposed on broad peaks due to amorphous carbons. All of the samples contain a significant organic component, with the LOI ranging from 64 to 87 % for the PM{sub 2.5} fraction and from 88 to 97% for the PM{sub 2.5+} fraction. {sup 13}C nuclear magnetic resonance (NMR) analysis indicates that the carbon is predominantly condensed in graphitic structures. Aliphatic structure was detected in only one of seven samples examined.