Limited information is available on the nature of organic compounds in the tropospheric aerosol and their effect on aerosol hygroscopic properties and cloud condensation nuclei (CCN) ability. Here we analyze samples of liquid droplets and interstitial aerosol, concurrently collected during fog episodes, to determine how the organic compounds are partitioned between the two reservoirs. By comparing the nature and concentration of different organic carbon classes found in the two reservoirs, we find that fog acts as an efficient separator for carbon (C) species on the basis of their chemical properties, with polar water soluble species representing the greater part of total C within fog droplets, and water insoluble C species preferentially found in the interstitial reservoir. Water-soluble organic species are scavenged by fog droplets to a comparable extent to major inorganic ions and are therefore expected to play an important role in the droplet nucleation process. The main classes of water soluble organic carbon (WSOC) identified in fog water and interstitial aerosol by the techniques traditionally used in aerosol analysis are aliphatic dicarboxylic acids, sugars, aliphatic alcohols, and aliphatic carboxylic acids. However, such species, ~120 individual compounds, only account for a few percent (<5% on average) of total WSOC. A new class of water soluble macromolecular compounds (MMC), detected in aerosol samples from different areas of the globe, are found to constitute a large fraction (~40% on average) of WSOC in the fog system (fog droplets plus interstitial aerosol) and represent the main class of water soluble species identified. More than 50% of WSOC still remains undetermined. ¿ 1999 American Geophysical Union