The study deals with the oxidation behavior of fuel cladding under mixed steam-air atmospheres. Oxidation tests of Zry-4 were carried out at temperatures of 1373 and 1573 K. Post-test weight gain measurement along with metallographic examination were conducted to study separately the kinetics of the region where nitrides formed and the nitride-free region. The results of oxidation tests at 1273 and 1473 K published previously by the same authors are also addressed in the discussion. The weight gain coming from the nitride-free region was estimated employing one-dimensional finite difference oxygen diffusion model and measured thicknesses of the metallic part of the oxidized specimen, the columnar oxide and the oxygen stabilized α-Zr(O) as well as the fraction of the columnar oxide at the oxide/metal interface. Consequently, the weight gain related to formation of the porous mixed ZrN–ZrO2 regions has been assessed. The results suggested that the nitrides formed beneath the protective columnar oxide layer and that a major part of the weight gain related to formation of the porous mixed ZrN–ZrO2 regions was associated with the formation of ZrO2 from α-Zr(O).