The variations in the isotopic compositions of chlorine in evaporation-controlled saline lake brines were determined by using an improved procedure for precise measurement of chlorine isotopes based on Cs sub(2) Cl sup(+) ion by thermal ionization mass spectrometry. The results showed that variation in delta sup(37) Cl values in these evaporation-controlled brines are attributable to evaporation of brine accompanied by the deposition of saline minerals. The isotopic fractionation of chlorine between the deposited saline mineral and the co-existing brine caused the variation of delta sup(37) Cl values in the brine. In general the isotopic fractionation of chlorine in nature indicates enrichment of sup(37)Cl in the solid phase relative to sup(35)Cl. The reverse isotopic fractionation of chlorine in which sup(35)Cl is enriched in the solid phase, was observed to some extent during quick deposition under laboratory conditions as well as in nature. The mechanism of isotopic fractionation of chlorine during evaporation deposition was studied