Blend of oxygenated alcohol fuels to diesel has drawn great attention as a way to promote the use of bioderived alternative fuels and for reduction of soot emissions in direct-injection diesel engines. This study compares the mixture condition, combustion and sooting characteristics of ethanol-diesel blends (E30 and E50: 30% and 50% ethanol blend to diesel in volume respectively) to those of a diesel in diffusion flames under various injection and ambient conditions. The comparisons were made in terms of liquid-phase penetration, flame structure, ignition delay, flame lift-off length, mixture condition at flame lift-off location, and soot concentration (KL factor) that obtained using Mie-scattering, natural flame luminosity imaging and two-color method. The liquid-phase penetration of the ethanol-diesel blends was almost equivalent to that of diesel under diesel-like injection and ambient conditions. The ethanol-diesel blend (E30) showed longer ignition delay than diesel, but this difference became insignificant at high ambient temperatures over 1000 K. The flame lift-off length of the E30 was longer than that of diesel that formed less fuel-rich mixture of E30 at the flame lift-off location. The difference in mixture condition between E30 and diesel became larger with increase in injection pressure, but smaller with increase in ambient temperature. The E30 suppressed the soot formation near the flame lift-off location, and also promoted the soot oxidation process. This soot reduction potential of E30 became larger with increase in injection pressure, but insignificant at high ambient temperatures. Increasing ethanol blend ratio decreased the flame temperature slightly, but further enhanced the soot reduction potential. (C) 2013 Elsevier Ltd. All rights reserved.