The kinetic curves of oxidation of Zircaloy-4 exhibit a transition, which is a sharp increase in the oxidation rate when the oxide thickness reaches a critical value. The pre-transition stage is controlled by the diffusion of oxygen vacancies in the oxide layer. In the post-transition stage, oxygen or water vapour have an accelerating effect on the oxidation (whereas they have no influence during the pre-transition) and the oxide layer is damaged, with large cracks parallel to the metal/oxide interface and connected to the gaseous atmosphere by pores. Consequently, it is clear that the post-transition stage cannot be accounted for by the same mechanism as in pre-transition. In this paper, we propose a geometrical modelling allowing to describe the progressive transformation of the oxide layer during the transition. This model is based on a random appearance of pores (connected to the external surface) which leads to the transformation, from a pre-transition stage to the post-transition stage, of small sections so of the oxide layer (analogy with the models of thermal transformations of powders, involving the processes of nucleation and growth of a new phase). The model allows to describe the kinetic curves obtained for the oxidation by water vapour of Zircaloy-4.