The oxidation by water vapour of a zirconium based alloy, a ZrNbO alloy containing 1% of Nb, has been studied between 500 and 550 °C, the water vapour partial pressure ranging in 13-80 h Pa, using isothermal and isobaric thermogravimetry, and calorimetry. During gravimetry experiments, sudden changes (jumps) in temperature or water vapour pressure have also been performed. It comes out that the kinetic behaviour is different before and after the transition, even though the approximations of steady-state and rate-limiting step are justified in both stages: the influence of temperature jumps is greater in pre-transition, whereas the effect of water vapour partial pressure is more pronounced in post-transition (nevertheless, an accelerating effect is also observed before the transition). No influence of hydrogen partial pressure has been observed. Besides, the higher the Nb content in the alloy, the higher the oxidation rate (in pre-transition). A mechanism has been proposed to account for the results obtained in pre-transition, involving the diffusion of adsorbed species in the porous part of the oxide layer as rate-determining step. The transition is accompanied by a change in the oxidation mechanism: in the post-transition stage, the kinetic curves being linear, the oxidation may be controlled by an interface step, which is probably different from the steps proposed for the pre-transition mechanism.