Relevance. The formation of groundwater various chemical types in the upper hydrodynamic zone (from HCO₃-Ca to Cl-Na) of southeastern Transbaikalia, with salinities exceeding 3 g/L. Existing hypotheses link this phenomenon to evaporation; however, this is not clearly observed in groundwater. Identifying geochemical water types through the lens of water–rock interaction could illuminate the mechanisms behind differing compositions and reveal stages of salt accumulation. Additionally, data on tritium content may aid in assessing the duration of solution contact with rocks and the water exchange intensity in the area. Aim. To identify the groundwater geochemical types in southeastern Transbaikalia and determine the necessary conditions for this based on isotopic-geochemical information. Objectsю. Groundwater from the upper dynamic zone (59 samples) in southeastern Transbaikalia and regional atmospheric precipitation (6 samples). Methods. Water major components were determined using titrimetric, potentiometric, and photometric methods, FAAS and FAES. Trace elements were analyzed using ICP-MS. Tritium concentrations were measured by liquid scintillation spectrometry. Saturation indices were calculated using the HydroGeo and GWB applications for physicochemical modeling based on the equilibrium constant method. Results and conclusions. The authors have identified three geochemical water types in southeastern Transbaikalia: siliceous, soda, and saline, along with some variations that sequentially replace each other as water moves from the mountainous framing to the Torey depression. This sequence reflects the evolutionary processes in the water–rock system depended on the process duration. Siliceous waters belong to the active water exchange zone, with a relative age of no more than 50 years and saturated with clays. Soda and saline waters exhibit slower exchange, with interaction times exceeding 57 years, facilitating saturation with more minerals, including calcite, while sodium accumulates in unsaturated solutions. Evaporation and mixing with saline lakes contribute to further salinization.