A boron isotope study combined with analyses of elemental boron, lithium, and chlorine is used to suggest that brines from the Dead Sea and on-shore hypersaline thermal springs (Hamme Yesha, Hamme Zohar, and Hamme Mazor) are the products of interaction of evaporated seawater with detrital sediments. The high δ11B values of the Dead Sea brines (55.7 to 57.4%. versus NBS-951) and the hot springs (52.2 to 55.7%.), and low B Li ratios (2.0 to 2.3 and 2.5 to 2.7, respectively), relative to seawater, indicate preferential removal of 10B from the brines and hence boron adsorption onto clay minerals. The brackish 'En Feshcha springs and the freshwater 'En Dawid and Nahal Arugot springs yield lower B contents and δ11B values (37.7 to 40.6%. and 33.8 to 36.9%., respectively). The δ11B values and B contents of diluted Dead Sea brines lie on calculated mixing lines between the composition of the brackish and freshwater springs with the composition of the Dead Sea. The δ11 B values of the hot springs, however, given their boron content, are significantly lower than those of the mixing lines. Thus, waters from the hot springs cannot be a mixing product of the Dead Sea brine with freshwater. Instead, the Dead Sea brine has evolved from the brines of the hot springs through further isotopic fractionation and boron adsorption onto detrital sediments.