The detailed kinetic mechanisms for the combined endothermic pyrolysis and steam gasification of petroleum coke are formulated using elementary chemisorption and reaction steps. The pyrolysis model is based on a linear combination of first-order decomposition rates of pseudo components. The gasification model is based on the oxygen-exchange mechanism describing reversible O-transfer surface reactions followed by a unidirectional gasification step, and on an extended mechanism describing, in addition, reversible steam sorption as OH/H groups and irreversible surface chemistry. The kinetic rate laws of Langmuir-Hinshelwood type are derived by assuming sorption equilibrium and by considering inhibition due to recombination of adsorbed oxygen and competition for active sites. The Arrhenius parameters of the rate constants for the pyrolysis and gasification of Flexicoke and Petrozuata Delayed coke are experimentally determined by dynamic thermogravimetric analyses with Ar, H2O, CO2, and H2O-CO2 mixtures in the range 500-1520 K.