Examination of boron isotopes, elemental B, Br, and Li in brines, and coprecipitated salts during fractional evaporation of sea water shows that Br, Li, and B in the evaporated sea water have lower concentrations than expected, as determined from mass-balance calculations. The deficiency is found beyond a degree of evaporation of ∼30 and is associated with a gradual increase in the δ11B values of the evaporated sea water, from 39‰ to 54.7‰ (relative to standard NBS 951). The high δ11B values of the brines and the relatively lower δ11B values of the coexisting precipitates (MgSO4 and K-MgSO4 salts; δ11B = 11.4‰ to 36.0‰) suggest selective uptake of 10B by the salts. Applying Rayleigh distillation equations, the empirical fractionation factors for the depletion of the salts in 11B are estimated as 30‰ (α = 0.969) for the early stages of precipitation (gypsum and halite range) and 20‰ (α = 0.981) for the late stages (K-MgSO4 minerals). Coprecipitation of B(OH)4- species with the salts, and/or precipitation of Mg-borate minerals with a coordination number of 4 are the proposed mechanisms for boron isotope fractionation during fractional evaporation of sea water. The boron isotope composition of sea water (δ11B = 39‰) is significantly higher than that of continental water (δ11B = -3‰ ±5‰). Our study shows that salt deposits may be depleted in 11B by 20‰ to 30‰ relative to their parent brines. These variations suggest that boron isotopes can be used to determine the origin (marine vs. nonmarine) of brines and ancient evaporitic environments.