To obtain a deep understanding of the pyrolysis mechanism of oil shale kerogen, critical organic intermediates were firstly prepared by heating Huadian oil shale up to the critical temperature point (similar to 350 degrees C), and then analyzed using Gas Chromatograph-Mass Spectrometer (GC-MS), Fourier Transform Infrared Spectrometer (FT-IR), Nuclear Magnetic Resonance Spectrometer (NMR) and X-ray Photoelectron Spectroscopy (XPS), etc. According to the results, there are about 84.3 aliphatic carbons per 100 carbons in the critical organic intermediates, less than that in the kerogen matrix studied previously, showing that the aliphatic carbons will decrease during thermal decomposition from kerogen to the critical organic intermediates. This might be mainly attributed to the breakup of aliphatic carbon chains and the resulting hydrocarbon gas emissions. Meanwhile, aromatic carbon structures are well retained in the critical organic intermediates, resulting in an increase in the aromaticity. In addition, the distributions of some heteroatoms (O, N and S) present in the critical organic intermediates were discussed as well. These results will be important for both the exploration of pyrolysis mechanism of kerogen and the improvement of quality and yield of shale oil. (C) 2013 Elsevier Ltd. All rights reserved.