The hygroscopic properties of submicrometer aerosol particles were studied with a Hygroscopic Tandem Differential Mobility Analyzer (H-TDMA) in the remote marine tropospheric boundary layer (MBL) over the Pacific and Southern Oceans in connection with the southern hemisphere marine First Aerosol Characterization Experiment (ACE 1) in October--ecember 1995. The H-TDMA was placed on board the ship R/V NOAA Discoverer and measured the hygroscopic diameter growth of individual aerosol particles when taken from a dry state to a relative humidity (RH) of 89--90%. Measurements were performed for the particles with dry diameters of 35, 50, 75, and 150 (or 165) nm. The natural aerosol present in the remote MBL largely consists of two types, a sea-salt component and a non-sea-salt (nss) sulfate component. Since their hygroscopic behavior is significantly different, the H-TDMA could clearly distinguish between these two types and thus make in situ measurements of the mixing state of the MBL aerosol. During the ACE 1 intensive campaign in the Southern Ocean south of Australia, the hygroscopic diameter growth factors at RH=90% for the nss-sulfate aerosol particles were 1.62, 1.66, and 1.78 at dry particle diameters of 35, 50, and 150 nm, respectively, and for time periods with remote marine air masses. These values exceed those normally found in continental polluted environments. The growth factors for the externally mixed sea-salt particles were even higher (2.12 and 2.14 for 50 and 150 nm). The corresponding values for the Pacific Ocean (at RH=89%) for the nss-sulfate particles were 1.56, 1.59, 1.61, and 1.63 for 35, 50, 75, and 165 nm. Particle deliquescence and RH hysteresis between RH=68--90% was only observed in air masses north of the South Pacific Gyre, and then only for the Aitken mode particles (particle diameters ~20--80 nm). The occurrence of externally mixed sea-salt particles could be linked to conditions with high wind speeds in connection with frontal passages or low pressure systems. Nevertheless, the number of externally mixed 150 nm sea-salt particles was found to be poorly correlated with local wind speed, probably due to a rather long life-time of these submicrometer particles. Particles with hygroscopic growth factors significantly less than those of the nss-sulfate particles (denoted less hygroscopic particles) were only present during periods with anthropogenic influence. ¿ 1998 American Geophysical Union