Stable chlorine (δ³⁷Cl) and bromine isotopes (δ⁸¹Br) are increasingly used to trace salinity sources and geochemical history of coastal brines. However, their application in hydrogeological models requires a more precise understanding of fractionation mechanisms. While Cl isotope fractionation during halide mineral precipitation and dissolution is well established, its behavior prior to halite saturation remains poorly understood. Furthermore, the isotopic behavior of Br remains considerably less characterized than Cl. By establishing a systematic baseline for changes in Cl and Br isotopic values during seawater evaporation and mineral precipitation, this study aims to refine their application in complex saline systems. Here, the evolution of δ³⁷Cl and δ⁸¹Br across brine, precipitates, and gas phases were tracked during controlled synthetic seawater evaporation. Our results show that both isotopes undergo measurable fractionation well before halite saturation, driven by partitioning into early carbonate and sulphate precipitates. While ³⁷Cl enrichment in precipitates relative to brine generally aligns with equilibrium lattice substitution, δ⁸¹Br shows a complex trend driven by non-equilibrium processes, such as kinetic entrapment of brine. Direct evidence of preferential ⁷⁹Br volatilization in gas phases also can contribute to enrichment of ⁸¹Br in residual brine and is potentially significant at the field scale. These findings reveal that halogen isotope evolution in multi-ion brines arises from a complex interplay of chemical partitioning, physical entrapment, and atmospheric losses. By establishing a systematic baseline for fractionation processes, this study offers improved constraints for assessing coastal brine evolution.