Surface segregation of NaBr in mixed NaBr/NaCl crystals precipitated from aqueous solutions containing Br is investigated. Crystals were grown from solutions of a wide range of Br:Cl values and studied using x-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), energy-dispersive x-ray fluorescence (EDX), and atomic force microscopy (AFM). Results indicate that NaBr surface-segregates to cover 4--5% of the surface at the Br:Cl value found in sea water (1:660). The surface NaBr:NaCl values are 35 times higher those of the bulk crystals. The surface NaBr forms domains of average size 635 nm¿635 nm¿53 nm. Large areas of the surface contain NaCl with no detectable NaBr. Complete surface segregation of all Br found in sea salt particles is likely to occur, thereby making a greater amount of Br available for surface reactions with gas-phase constituents than would be predicted based on bulk composition. ¿ 2001 American Geophysical Union