The paper deals with formal thermodynamic modeling of uranium in-situ leaching (ISL) processes and the optimization of technological schemes. Presently, for the uranium extraction from ore, the ISL is used, i.e. its selective dissolution by a chemical reagent in the ore body, followed by subsequent removal of formed chemical compounds from the reaction zone by the moving stream of solvent. Pre-comminuted raw ore is leached with sulfuric acid (acid leaching) or carbonate solutions (saline leaching) in the presence of an oxidant for transferring uranium into the solution as sulfate or carbonate complexes of U(VI). In the case of ISL, carbonate solutions are mostly used without polluting the environment. Yet, carbonate leaching is more selective to uranium than acid one. As the oxidizing agent H2O2, oxygen, nitrate ions and alkali metal hypochlorite are mainly used. At the carbonate leaching of uranium from its ores, alkaline solutions are obtained, where uranium is in the form of complexes UO2(CO3)4- 3 .For the extraction of uranium from carbonate solutions, its precipitation in the form of uranium peroxideUO UO4 H O 4 2 ⋅ is largely used. Since the pitchblende group, which constitutes the bulk of industrial uranium ores, is characterized by the 4- 3 general composition UO x 2+ , where 0 ≤ x ≤ 1, in which U(IV) is oxidized to U(VI) up to various degrees, for the thermodynamic modeling of ISL processes the dissolution of minerals of the composition UO2 has been studied. The process of UO2 dissolution in the carbonate medium is usually described by the equation: 2 UO O CO HCO UO (CO H O S 2 4 3 2 3 3 2 2( ) 3 +[ ] + + 2 = + − − − (1), where [O] is an oxidant. The precipitation of uranium by acidification of carbonate solutions with 2 2 H O can be represented as follows: UO + + H O +nH O =UO ⋅ nH O + H+ S 2 2 2 2 4 2 ( ) 2 2 (2) However, equations (1) and (2) do not sufficiently describe various chemical processes in these systems. Indeed, by varying the experimental conditions (pH, total concentrations of reagents et al.), the composition and the stability of particles present in the system are changing. Thus, the composition and stability of the carbonate complexes UO2(CO3)i UO (CO ) 2 3 strongly dependent on pH and the total carbonate concentration in the solution. By increasing the pH, a significant role is played by the hydrolysis of uranyl ion. Furthermore, in the presence of H202 the UO2+ 2 UO ions form sufficiently stable j UO (O ) 2 2 complexes. Finally, the possibility of the reaction (1) depends on the nature of the oxidant. For full describing of the dissolution of the mineral (solid phase), the generalized equation (GE) of interaction of all the particles present in solution has been used [1]. Within the approach based on GEs of reactions, the thermodynamic calculation of the optimal conditions of underground leaching of uranium ores, based on the model UO2(S ). UO , has been carried out. The thermodynamic stability area of the mineral has been determined by the value of the Gibbs energy of the overall process [1-4], described by the GEs. The solid-phase is stable if ΔGS,tot > 0. The condition ΔGS,tot = 0 corresponds to the beginning of its dissolution and (or) sedimentation. It is shown that the most optimal oxidants from the thermodynamic point of view, there are hydrogen peroxide and alkali metal hypochlorite. The obtained results of modeling within the formal chemical thermodynamics are in good agreement with the existing experimental data and technological schemes for the ISL of uranium ores.