The so-called induction period, the time delay between the initial exposure to oxygen of an oil or oil fraction and the start of rapid oxidation, was examined experimentally for the saturates fraction separated from a Lloydminster heavy oil. The observed kinetics could be explained by assuming that the saturates contained a small amount of naturally occurring oxidation inhibitors, which repressed the oxidation rates by rapidly consuming an essential intermediate in the reaction chain, but which were also gradually consumed in the process. This observation explains some of the complexity that has been seen in the oxidation rates that control combustion front development during in-situ combustion, and provides some added direction in the development of a comprehensive reaction model for this process.