Isothermal oxidation experiments with cladding tube segments of Zircaloy-4 (Zr-1.3%Sn) in oxygen–nitrogen model mixtures were performed at 800, 1000, and 1200 °C for 6, 1 h, and 15 min, respectively. The gas compositions varied between 0 and 100 vol% nitrogen including 1 and 99 vol%. A strong accelerating effect of nitrogen on the oxidation kinetics was seen for a wide range of boundary conditions. At 800 °C, oxidation in all mixtures with 1–99 % nitrogen resulted in higher reaction rates compared to the pure gases, especially after transition from protective to non-protective oxide scales. At 1000 and 1200 °C, only starvation of oxygen in mixtures with low oxygen contents resulted in lower rates compared to pure oxygen. The oxide scales formed in the mixtures were very porous due to the formation of zirconium nitride at the metal-oxide interface and its oxidation during continuing reaction. The extension of the oxide-nitride zone increased with temperature and with nitrogen content in the gas mixture. Nitrogen seems also to affect the pre-transition reaction kinetics. The mechanism of the faster oxidation kinetics of zirconium alloys in atmospheres containing nitrogen will be discussed in this paper.