Using a laser ionization mass spectrometer, the chemical compositions of single particles larger than 0.16 &mgr;m diameter were measured in the marine boundary layer (MBL) at Cape Grim, Tasmania during the ACE-1 experiment. Chlorine was observed in nearly all of the particles. Bromine was internally mixed with amounts near the sea water mole fraction in most particles. The aerosol mole fraction of iodine was similar to bromine, so iodine was highly enriched relative to sea water. Iodine was not uniformly mixed among all particles. Iodine-rich particles included sea salt and metals. Iodine was also correlated with the presence of organics in the spectra. Chlorine, bromine and iodine were anti-correlated with sodium sulfate. Since particles accumulate sulfate in the MBL, this anti-correlation suggests that the aerosols were losing all three halogens as they aged. ¿ 1997 American Geophysical Union