The dried sewage sludges were pulverized and combusted in an oxygen-enriched atmosphere to investigate the emission of particulate matter smaller than 1.0 mu m (PM1). Combustions were conducted at 1200 degrees C in a drop tube furnace with the oxygen content varying from 10% to 30% and 50% in the gas atmosphere. The collected PM1 was characterized including its concentration, particle size distribution, and the size-dependence of the elemental composition and chemical species. The results indicate that, after the complete combustion of sludges, the PM1 formed accounts for about 1 similar to 7% on the basis of total ash, which is influenced significantly by the oxygen content in the gas atmosphere as well as the sludge type. In general, because of the improvement on the temperature of char particles and their breakage, the amount of PM1 is increased exponentially with the increasing of the oxygen content. With regard to PM1 formation mechanisms, the refractory elements within it, including Si, Al, Ca, Fe, and Mg, undergo two pathways for their transformation: vaporization-condensation for their presence in the particulates <= 0.1 mu m (PM0.1) and direct liberation of the inherent fine minerals for their presence in those > 0.1 mu m. Their modes of occurrence in raw sludge play an important role in the vaporization. At 10% oxygen content, decomposition of the organically bound fraction is the main way contributing to their presence in PM0.1. With the oxygen content increases, more of the inorganic species vaporizes too. Direct liberation of the refractory metals is also determined positively by the oxygen content in the gas atmosphere, which is due to the breakage of char particles at an elevated oxygen content. With regard to the volatile elements in PM1, including P, S, Na, K, Zn, Cl, and Mn, they were mainly generated by the vaporization-condensation mechanism. Vaporization of S, Cl, Na, and K is significantly obvious at 10% oxygen, and hence, sulfates and chlorides are favored. With the oxygen content increases, the vaporization of P and Zn becomes significant. The amounts of Na and K are almost unchanged; meanwhile, S and Cl decrease somewhat. Formation of the phosphates is facilitated, owing to the high partial pressure of P. A portion of Na and K is possibly captured by Al silicates. Zn is also possibly captured by apatite and iron oxide to form the complex compounds. The toxicity of these species needs a detailed evaluation.