Stable and radiogenic isotope compositions are commonly used to identify the solution from which minerals such as sulphate, carbonate and silicate were crystallized. Both the sources and the ages of evaporite deposits can be determined by measuring radioactive 36Cl concentrations using new developments in accelerator mass spectrometry. Halite crystals found in saline sediments, which could not be measured by isotope techniques until this method was developed, are usually assumed to have precipitated from the lake water during former dry periods. In the case of the Dead Sea we tested this assumption by comparing the abundance of 36Cl isotopes in a shallowly buried halite layer with that in the overlying water. We found that the 36Cl/Cl ratio is significantly lower in the halite, which indicates that the salt did not crystallize from this water but from a solution originating from the Tertiary diapir underneath the sea or that the salt represents the top of the Mt Sdom formation diapir. �� 1986 Nature Publishing Group.