The current goal in uranium mining is to ensure the maximum effect at minimized costs. One of the problems when using the in-situ leaching (ISL) technology consists in the incomplete mining of the process blocks due to premature uranium content reduction in the pregnant solutions, leading to increased mining times and excessive reagent consumption. The required complete block mining shall be ensured with the highest possible efficiency. The ISL technology is currently used for the mining of all hydrogenous uranium deposits in the Republic of Kazakhstan, which are often characterized by complex geological and hydrogeological conditions. One of these deposits, Semizbay, rates as complexity type III. The conditions at the deposit require increasing the redox potential of the solution by oxidizing Fe2+ into Fe3+. The article considers the addition of hydrogen peroxide to the leaching solutions to improve the redox potential with due account of the mining and geological features of the Semizbay deposit. The pilot tests have established that this oxidizing agent has a positive effect on the uranium content in the pregnant solution with the concentrations of 0.2–0.3 l/m3, provided that the concentration of sulfuric acid in the leaching solutions is within the range of 13–15 g/l. The studies allowed increasing the concentration of uranium in the pregnant solutions from 74 to 114 g/l (i.e., by over 50 %). The authors are grateful to the staff of the geotechnology department of the Semizbay mine, Semizbay-U LLP, for collecting the data and to the staff of the IVT-Zerde LLP laboratory (branch of the Semizbay mine) for the high-quality laboratory experiments during the pilot tests at the mine.