Vegetable oils are candidates for alternative fuels in diesel engines. These oils, such as soybean, sunflower, rapeseed, cottonseed, and peanut, consist of various triglycerides. The chemistry of the degradation of vegetable oils when used as alternate diesel fuels thus corresponds to that of triglycerides. To study the chemistry occurring during the precombustion phase of a vegetable oil injected into a diesel engine, a reactor simulating a diesel engine was constructed. Pure triglycerides were injected into the reactor in order to determine differences in the precombustion behavior of the various triglycerides. The reactor allowed motion pictures to be prepared of the injection event as the important reaction parameters, such as pressure, temperature, and atmosphere were varied. Furthermore, samples of the degradation products of precombusted triglycerides were collected and analyzed (gas chromatography / mass spectrometry). Two series of experiments wereconducted at temperatures of up to 400°C and 450°C. In these experiments, a temperature increase from 400° to 450°C showed differences in the mate rials formed. At 450°C, differences in the behavior of the triglycerides under varying atmosphere were observed which were not detected at 400°C. Data show that the triglycerides were rapidly degraded to numerous compounds. The experiments and their results are discussed here.