Mean activity coefficients of NaCl, KCl and MgCl 2 have been calculated for the chloride brines of the Dead Sea (ionic strength 7.9-8.8). The values of the activity coefficients have been used to calculate the solubility of halite and carnallite (KMgCl 3· 6H 2O) in the Dead Sea brine. The lower water mass of the Dead Sea (approximately 56 per cent of the lake volume) is, within the errors of the method, at equilibrium with halite. This result is in agreement with the occurrence of halite on the lake bottom. Evaporation of the lake with resultant increase in the concentration of K + and Mg 2+ leads to precipitation of carnallite when the concentrations have increased by a factor of approximately 1.85. Dilution of the Dead Sea brine, however, would result in an increase in the concentration of NaCl in the brine due to dissolution of the halide layer present on the bottom. Using the published solubility data for the system KCl-MgCl 2-H 2O and the calculated values of the activity coefficients and the activity of H 2O, the dissociation constants of carnallite and bischofite have been estimated at 25°C as logKcar = 4.00 ± 0.05 and logKbisch = 4.445. The values of the activity of water calculated for some natural brines agree to within 2-4 per cent with data reported in the literature. Using the methods of calculation discussed in the paper it has been shown that the ground water brines from the Dead Sea coastal areas are close to saturation with respect to halite, yet they are undersaturated with respect to sylvite and carnallite. This suggests that the Dead Sea has not evolved through a stage of brines more concentrated than the present.