Technetium-99 is a mobile, long-lived radionuclide and environmental risk driver at some nuclear waste sites. The feasibility of decreasing 99Tc mobility in vadose zone sediments using H2S and NH3 gases was evaluated in laboratory experiments. In untreated sediments, 75 to 95% of the 99Tc was leachable. Using combinations of H2S and NH3 gases, the 99Tc mobility was reduced to 14 to 48%. Individual H2S or NH3 gas treatment of sediments had little lasting effect. For the combined gas treatment, the H2S gas created reducing conditions at the pore water–mineral interface, which temporarily reduced and precipitated 99Tc, while the NH3 gas created alkaline pore water that caused mineral dissolution. As the pH neutralized, subsequent aluminosilicate precipitation probably coated 99Tc precipitates and rendered them less mobile. Surface phase analysis showed that 99Tc was associated with weathered basalt clasts and S, possibly from the precipitation of TcSx. Treatment performance was nearly the same at different 99Tc concentrations (1.3–240.5 Bq g−1), water contents (1–8%), and gas injection rates but was sensitive to gas concentrations. Low gas concentrations (<3%) had insufficient reductant or slower mineral dissolution. High gas concentrations (>30%) formed an NH4SH precipitate. The 14 to 48% mobile 99Tc remaining after gas treatment may have been caused by the limited time for aluminosilicates to precipitate in our experiments. Degradation of added NH3 was not observed during the 3-mo experiment. Overall, this study showed that combined H2S and NH3 gas treatment of low-water-content sediments can be applied to significantly decrease 99Tc mobility.