—The paper presents the formation of the chemical composition of water in the sulfate mineral lakes of western Transbaikalia. It is shown that sodium sulfate water in the regional lakes forms in two zones of continental salinization, the Barguzin and Gusinoe Ozero–Orongoi depressions. The average mineralization of the lakes reaches 30.8 g/L, pH = 7.90–8.84, the fractional content of sulfate ion is 55.2–81.8% eq, and the fractional content of sodium ion is 59.1–82.1% eq. The paper presents the major-ion and trace-element composition of lake waters and the chemical and mineral composition of bottom sediments. It is shown that groundwater enriched in sodium sulfate plays a major role in the lake formation.