Mobility of hexavalent uranium [U(VI)] in H2S-treated soils was investigated using laboratory column experiments to assess the potential of applying in situ gaseous reduction for U immobilization in the vadose zone. Soil from the Hanford Formation in the U.S. Department of Energy Hanford Site, Washington, was used in this study. The impact of water chemistry and soil treatment on U(VI) immobilization and the role of gas humidity on soil treatment were investigated. The study revealed that soil uptake of U(VI) from deionized water was much higher than that from the simulated Hanford groundwater. Nevertheless, gas-treated soil was still shown to have the potential for immobilizing U(VI) from the simulated groundwater. In addition, changes in H2S column breakthrough indicated that humidity enhanced the reduction of soil Fe. In the first 20 pore volumes, the soil treated with moisturized H2S gas can effectively immobilize >80% of the mobile U(VI). Primary mechanisms for U immobilization included U(VI) sorption to the sediments, reduction of U(VI) to insoluble U(IV), and enhanced adsorption of U(VI) to newly formed Fe oxides. Remobilization of U following reoxidation of the sediment was relatively insignificant under the experimental conditions applied, apparently owing to the enhanced adsorption of U to poorly crystallized hydrous ferric oxide products.