Theoretical calculations are presented on elementary reactions which are important during coke formation in a thermal cracking unit. This process is known to proceed through a free radical chain mechanism. The elementary reaction steps that lead to the growth of the coke surface can be divided into five classes of reversible reaction: hydrogen abstraction; substitution; gas phase olefin addition to radical surface species; gas phase radical addition to olefinic bonds; and cyclization. To identify the elementary reaction classes that determine the coking rate, all microscopic routes that start from benzene and lead to naphthalene have been investigated. It is found that initial hydrogen abstraction reactions and subsequent addition reactions determine the global coking rate. To determine the influence of the local polyaromatic structure on the kinetics of the hydrogen abstraction reactions, calculations on polyaromatic aromatic hydrocarbons (PAHs) have been performed. This has revealed six types of possible reactive site at the coke surface, with the local polyaromatic structure strongly influencing the reaction kinetics. Overall, we find that hydrogen abstraction preferentially occurs from the surface sites of smaller PAHs, such like benzene, naphthalene,…