The kinetic behavior of Zircaloy-4 tube samples was examined during high temperature steam oxidation in order to propose a mechanism with detailed elementary steps. The study revealed that steam and hydrogen partial pressures have no effect on the reaction kinetics. Based on these experiments, the kinetic model established herein takes into account the oxide growth and the oxygen dissolution in the metal. A ratedetermining step of diffusion of oxygen vacancies in the oxide governs the kinetics of the reaction in both pre- and post-breakaway regimes meaning that the oxide growth and oxygen dissolution in the metal advance at proportional rates.