Abstract: Under anoxytropic conditions, source rock samples were heated tosim-ulate their thermal evolution. The light hydrocarbon compounds (C₅-C₁₀) extracted from the pyrolysis products using low boiling point solvent are defined as light hydrocarbon pyrolysates.Gas chromatography shows that there are more than 100 compounds in light hydrocarbon pyrolysates. With the view of chemical thermodynamics, the generation, disappearance and intertransformation of light hydrocarbons are closely related to the source materials, the conditions and thermal environments of organic matter. By the determinations on source rock samples with five organic types, we draw some conclusions that the compositions of light hydrocarbon pyrolysates will vary with the sedimentary enviroments of mudstone, organic types and the degree of thermal evolution. Several trial parameters are also proposed in the paper.