The chemical composition of lake water and inflowing river water was investigated in Nam Co (lake), Tibetan Plateau, in September 2005. Lake water samples (n = 76) were collected at different depths along a south-north transect of the eastern part of the lake while water samples were collected from 69 rivers flowing into the lake; a sediment core was collected at 64 m midway along the water survey transect to investigate salt precipitation. Na⁺ and Ca²⁺ were the dominant cations, accounting for 76.2% and 60.6% of the lake and river water cations, respectively while was the dominant anion accounting for 70.8% and 93.4% of lake and river anions, respectively. CaCO₃ precipitation from the water column decreased Ca²⁺ in the lake water, with the relative proportion of other ions increasing significantly. Evaporation-crystallization processes largely control Nam Co lake-water chemistry, while rock weathering is the dominant processes influencing the chemistry of river water; carbonate and silicate weathering are the major sources of ions in these rivers.