As the world's oil reserves are limited, and as a partial mitigation of greenhouse gas emission, renewable biofuels are being considered as important contributors to the future fuel supply for the transportation sector. The combustion of biofuel-diesel mixtures in practical engines has been shown to be not only feasible but also favorable due to low particulate emission characteristics. This paper demonstrates quantifiable sooting propensity of biofuel-diesel fuel blends using classical smoke point observations and laser induced-incandescence and laser extinction optical methods. In particular, we study mixtures of 0-25% by volume of soybean biofuel in ultra-low sulfur diesel. Following the ASTM D1322 standard, we find that the maximum flame height at the smoke point condition increases linearly with increasing biofuel fraction. An alternative sooting propensity measurement is needed, however, because high bio-fuel/diesel blends do not produce a smoke point in the standard wick-fed lamp procedure. Using a fixed flame height, laser-based measurements are generally consistent with smoke point trends, and laser extinction calibrations provide quantitative soot volume fractions. The results show the greatest soot concentration for pure diesel fuel, B0, and the least for a 20% blend by volume of biofuel, B20. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.