This paper deals with the oxidation behavior of Zry-4 nuclear fuel cladding tubes in mixed steam–air atmospheres at temperatures of 1273 and 1473 K. The main goal is to study the oxidation kinetics of Zry-4 fuel cladding in dependence on the air fraction in steam in the range from 0% up to 100%. The purpose of this study is to provide experimental data suitable for an oxidation correlation, applicable for severe accident computer codes of nuclear power reactors. The influence of the air addition in steam on parameters of Zry-4 kinetic equation has been quantified using the results of weight gain measurements. At 1273 K, both pre-transition and post-transition regimes were treated. The results of weight gain measurements showed a strong dependence of the Zry-4 oxidation kinetics on the air fraction in steam, especially at 1473 and at 1273 K in the post-transition regime.