Most petroleum is formed through the partial decomposition of kerogen (an insoluble sedimentary organic material) in response to thermal stress during subsurface burial in a sedimentary basin(1,2). Knowing the mechanisms and kinetics of this process allows the determination of the extent and timing of petroleum formation, which, in turn, are critical for evaluating the potential for petroleum occurrences within a sedimentary basin. Kinetic models of petroleum generation are derived mainly from pyrolysis experiments(1,2), in which it is usually assumed that formation rates are controlled by the strength of the bonds within the precursor compounds : this agrees with the observation that petroleum formation rates increase with increasing sulphur content of thermally immature kerogen(2-4), C-S bonds being weaker than C-C bonds. However, this explanation fails to account for the overall composition of petroleum, Here I argue, on the basis of pyrolysis experiments, that it is the presence of sulphur radicals, rather than the relative weakness of C-S bonds, that controls petroleum formation rates, My findings suggest that the rate of petroleum formation depends critically on the concentration of sulphur radicals generated during the initial stages of thermal maturation. The proposed mechanism appears to provide a realistic explanation for both the overall composition of petroleum and the observed variation in formation rates.