The combustion characteristics and soot distributions of neat butanol and neat soybean biodiesel were explored in an optical constant-volume combustion chamber using natural flame luminosity and forward illumination light extinction methods under various ambient temperatures (800 and 1000 K) and oxygen concentrations (21%, 16%, 10.5%). Results demonstrated that butanol had a higher normalized peak pressure compared with the biodiesel. Soybean biodiesel autoignited earlier than butanol at 21% and 16% oxygen, while a reverse trend was found at 10.5% oxygen. The oxygen concentration had little effect on the autoignition timing for butanol when it was between 16% and 10.5% at 800-1000 K. The natural flame luminosity reduced with lowered oxygen concentration and the flame distribution was notably increased at 10.5% oxygen. At 800 K ambient temperature, there was no soot formation detected for butanol, while the net soot release for soybean biodiesel was reduced with the decrease of oxygen concentration. At 1000 K ambient temperature, the net soot release increased for both butanol and soybean biodiesel with the decrease of oxygen concentration. Compared with butanol, soybean biodiesel had a higher value of normalized time integrated soot mass (NTISM) under all conditions. The value of NTISM increased about 16 times from 800 to 1000 K for the soybean biodiesel at 10.5% oxygen, indicating that low oxygen concentration will deteriorate combustion and increase the difficulty of soot emissions control for the soybean biodiesel under a higher ambient temperature.