Reformulation of petroleum diesel through its blending with various oxygenated fuels has been explored for reducing emissions of diesel particulate matter (DPM). Such oxygenated fuel blends tend to alter the physicochemical characteristics of DPM and its toxicity due to changes in the combustion chemistry, caused by the thermo-chemical properties of the oxygenated fuels. However, there is a lack of in-depth investigations on the changes in the chemical composition of size-segregated DPM and its related toxicity, especially for smaller particles, when the blended fuels are combusted in diesel engines. To fill this knowledge gap, we examined the effects of blending ultralow sulfur diesel (ULSD) with five different oxygenated fuels (diglyme (DGM), palm oil methyl ester (PME), dimethyl carbonate (DMC), diethyl adipate (DEA), and butanol (Bu)) at 2% and 4% oxygen levels on the carbonaceous composition of size-segregated engine-out DPM and the toxicity of quasi-ultrafine particles (quasi-UFPs, aerodynamic diameter < 0.2 mu m). The oxygenated fuel blends are effective in promoting soot oxidation in all size ranges and in reducing the total engine-out DPM mass concentrations, but lead to a significant increase in organic carbon (OC) fractions. Smaller particles had higher fractions of OC, particle-bound PAHs and n-alkanes, which varied with the type and content of oxygenates used. The blended fuels significantly affected the surface areas of both soot and volatile particles in the quasi-ultrafine size range, the cytotoxicity of the quasi-UFPs, and altered the global gene expression with a broad range of biochemical pathways due to the changes in the engine-out particle composition caused by the blended fuels.