This paper presents measurements of the soot temperature and KL factor for biodiesel and diesel combustion in a constant volume chamber using a two-color technique. This technique uses a high-speed camera coupled with two narrowband filters (550 nm and 650 nm, 10 nm FWHM). After calibration, statistical analysis shows that the uncertainty of the two-color temperature is less than 5%, while it is about 50% for the KL factor. This technique is then applied to the spray combustion of biodiesel and diesel fuels under an ambient oxygen concentration of 21% and ambient temperatures of 800, 1000 and 1200 K. The heat release result shows higher energy utilization efficiency for biodiesel compared to diesel under all conditions; meanwhile, diesel shows a higher pressure increase due to its higher heating value. Biodiesel yields a lower temperature inside the flame area, a longer soot lift-off length, and a smaller soot area compared to diesel. Both the KL factor and the total soot with biodiesel are lower than with diesel throughout the entire combustion process, and this difference becomes larger as the ambient temperature decreases. Biodiesel shows approximately 50-100 K lower temperatures than diesel at the quasi-steady stage for 1000 and 1200 K ambient temperature, while diesel shows a lower temperature than biodiesel at 800 K ambient. This result may raise the question of how important the flame temperature is in explaining the higher NOx emissions often observed during biodiesel combustion. Other factors may also play an important role in controlling NOx emissions. Both biodiesel and diesel temperature measurements show a monotonic dependence on the ambient temperature. However, the ambient temperature appears to have a more significant effect on the soot formation and oxidation in diesel combustion, while biodiesel combustion soot characteristics shows relative insensitivity to the ambient temperature. (C) 2013 Elsevier Ltd. All rights reserved.