Four bituminous coals, possessing similar organic properties, were combusted in a lab-scale drop tube furnace (DTF) to investigate the emission of particulates less than 1.0 mu m in diameter (PM1). The combustion conditions are as follows: 1450 degrees C, air as gas atmosphere, and a residence time of about 3 s. The results indicate that PM1 is formed by two pathways: metallic vaporization and direct liberation of inherent submicrometer particles. Excluded minerals play no role in its formation. The amount of PM1 as well as the concentrations of individual elements within it vary with coal considerably. Regarding the refractory elements, the transformation of Si and Al is likely affected negatively by the amount of excluded (CaO+Fe2O3) in the parent coals. Ca and Mg in PM1 are entirely contributed from their organically bound fraction and the inherent submicrometer particles containing these two elements. A small amount of coarse particles containing Fe (> 1 mu m) also possibly transforms into PM1 via vaporization or fragmentation. Its extent is dependent on the mode of occurrence of Fe in raw coals. For the volatile elements in PM1, S, P, Na, and K are the most prevalent; they are mainly in forms of sulfates, phosphates, and P2O5. The amount of S is determined by the presence of alkali elements and Ca in PM1, due to their interactions. The other three are, however, affected by their original modes of occurrence. P in a complicated form containing Si, Al, Ca/Fe, and P likely vaporizes readily due to its melting propensity. The water-soluble species containing Na and K preferentially vaporizes too. The vaporized heavy metals, Mn, Ni, and Cr studied here, possibly bind with the refractory Si and Fe in PM1, which is proven by both the variation of their concentrations with particulate size and a thermodynamic equilibrium consideration.