There is a little previous literature on isotope geochemistry study of chlorine cycle in freshwater regions by far. We propose to carry out a study on the chlorine and sulfur isotope geochemistry of surface waters from three large rivers in China. We will quantify isotopic composition variations of chlorine, sulfur and oxygen of dissolved sulfate in surface water of major river basins with different climate, geological and geographical backgrounds. The characteristics of the isotopic variations will be used to identify the sources of the related elements and compounds, to elucidate the factors controlling Cl and S cycling within river basins in different climatic zones, and to distinguish the natural versus anthropogenic sources. The expected results will provide new supporting scientific data on, and insights to, aquatic geochemistry of river basins. The study will advance the frontier of environmental chlorine isotope geochemistry.