Oxidation rates of Zircaloy-4 cladding tubes have been measured in hydrogen-steam mixtures at 1200 to 1700°C. For a given isothermal oxidation temperature, the oxide layer thicknesses have been measured as a function of time, steam supply rate, and hydrogen overpressure. The oxidation rates in the mixtures were compared with similar data obtained in pure steam and helium-steam environments under otherwise identical conditions. The rates in pure steam and helium-steam mixtures were equivalent and comparable to the parabolic rates obtained under steam-saturated conditions and reported in the literature. However, when the helium was replaced with hydrogen of equivalent partial pressure, a significantly smaller oxidation rate was observed. For high steam supply rates, the oxidation kinetics in a hydrogen-steam mixture were parabolic, but the rate was smaller than for pure steam or helium-steam mixtures. Under otherwise identical conditions, the ratio of the parabolic rate for hydrogen-steam to that for pure steam decreased with increasing temperature and decreasing steam supply rate. The smaller parabolic rates in the hydrogen-steam mixtures indicate that, under the test conditions, a considerable amount of hydrogen was dissolved in Zircaloy during oxidation and, as a consequence, oxygen transport in both the oxide and alpha-phases was slower than in the corresponding phases produced in pure steam or helium-steam mixtures. The findings are discussed in relation to some reported high-temperature transport characteristics of oxygen ions in the oxide formed in hydrogen-steam mixtures. For smaller steam supply rates (sufficiently small but still larger than the “steam starvation” limit), significantly smaller linear oxidation rates were observed in the hydrogen-steam mixture, which indicates that the oxidation rate was limited either by the gaseous transport of the steam molecules or as a result of chemical reaction between the chemisorbed oxygen on the zirconium oxide surface and the gaseous hydrogen molecules. This phenomenon of “hydrogen blanketing” is discussed in relation to fuel rod heatup in a degraded-core-accident situation.