The effect of alloying uranium with 7.5-wt% Nb and 2.5-wt% Zr (“Mulberry” alloy) on the initial oxidation kinetics (surface oxide thicknesses up to 2000 Å) in air containing water vapor was investigated using an ellipsometer high vacuum system. The uranium alloy samples were oxidized in dry air(<5 ppm H2O)and air containing water vapor(15 000 ppm H2O)at a pressure of 650 Torr in a temperature range of 22 °–200 °C. The results differed from the oxidation of uranium in that the oxidation of this uranium alloy was not enhanced by water vapor under the conditions studied. The oxidation of unalloyed uranium in air is accelerated by water vapor. The oxidation followed a parabolic-rate law, indicating a diffusion-controlled mechanism. An Arrhenius plot of the parabolic reaction rates as a function of the reciprocal temperature revealed a difference in the activation energy of the low-temperature (22 °–100 °C) and high-temperature (100 °–200 °C) oxidation.