In order to better understand chlorine isotopic variations during brine evolution, experiments were designed to determine the changes in the chlorine isotope composition (delta Cl-37 value) during evaporations of solutions containing NaCl, KCl and MgCl2 center dot 6H(2)O at 28 +/- 2 degrees C. Three evaporation experiments were conducted in a clean environment. The precipitate and brine samples were collected during the evaporation, and the chlorine isotopic ratios of the samples were determined using an improved thermal ionisation mass spectrometry procedure based on Cs2Cl+ ion measurement. The results are as follows: the mean fractionation factors of the three solutions are alpha(Na) = 1.00055, alpha(K) = 1.00025, and alpha(Mg) = 1.00012, respectively, where alpha(Na), alpha(K) and alpha(Mg) are the fractionation factors between salts (NaCl, KCl and MgCl2 center dot 6H(2)O) and saturated solutions. The results showed that the delta Cl-37 values of precipitate and coexisting brine samples decrease during the precipitation of single chloride minerals. The residual brine was a Cl-35 reservoir for different single chloride solutions. New chlorine isotopic evolution curve during seawater evaporation were also calculated. The results indicated that during the primary precipitation stage of halite, delta Cl-37 decreased continuously, and the most important thing is that this trend continues during the final stages when Mg-salts begin to precipitate.