This paper presents data on the major and trace element composition of saline lakes of western Mongolia. The main geochemical types of lakes distinguished in the study are soda-, chloride-, and sulfaterich lakes. Lake water equilibria with major carbonate, sulfate, chloride, and other rock-forming minerals were calculated. The results show that the major factors controlling the formation of each lake type include evaporation and water-rock interaction processes, and the latter factor plays a critical role in freshwater and soda lakes and only a minor role in chloride lakes. Special attention was given to the soda lakes and the factors controlling lake water chemistry.