The results of experiments to rapidly pyrolyze oil shale from the Green River Formation using a xenon flash lamp reactor are presented. A variety of analytical tools were used to determine the composition of the liquid products and to compare the rapid pyrolysis products with those from conventional retorting processes. The tools included infrared, proton nuclear magnetic resonance, high performance liquid chromatography and field ionization mass spectroscopy. The reaction mechanisms of rapid pyrolysis were found to be different from those of slow-heating rate processes. Thermal fragmentations were non-selective, resulting in a broad range of high molecular weight components that contained more heteroaromatic functionality than in the products of conventional retorting. Other reactions that were found to differ in slow versus rapid pyrolysis were dehydrogenation of hydroaromatic compounds and formation of condensed and noncondensed aromatic compounds. As the input energy to oil shale is increased, even though the oil shale was heated to high temperatures, the liquid products still contained significant amounts of alkanes and olefins. Based on these results, the high conversion of kerogen by rapid pyrolysis produces a low quality fuel which would require upgrading before utilization.