The composition of lake and reservoir waters in Taiwan is mainly controlled by inputs form chemical weathering and by concentration through water evaporation. Inuputs form seasalt aerosols are secondary. Sulfate in the water samples mostly originates from oxidation of sulfide minerals in source rocks, such as marbles, geniss and schists, in the Central Range of Taiwan. Pco2 values estimated form pH and alkalinity data are mostly higher than the atmospheric Pco2, indicating a net input of CO2 form biological respiration over photosynthesis in lakes and reservoirs. Those water samples are also undersaturated with respect to calcite. Some coastal water samples with high alkalinity have unusually high pH (or low Pco2) and are saturated or slightly oversaturated with respect to calcite, probably suggesting a net removal of CO2 by active photosynthesis along with evaporative concentration of alkalinity in these water bodies. Some high mountain lakes are low in alkalinity and may be susceptible to acidification by acid rain. However, exceptionally high chemical weathering rates in the Central Range may mitigate the acid rain problem. Kaolinite is a stable secondary mineral in high mountain lakes ( > 200 m), wheras Ca-smectite and /or kaolinite are stable secondary minerals in other water bodies. The chemical weathering rates of the Taiwan Island are extremely high, but the chemical compositions of lakes and reservoirs in Taiwan are, in general, similar to the composition of the world mean river water.