Three surface parameters - specific surface area (SSA), heat of wetting (Hw), and cation exchange capacity (CEC) - of estuarine suspended solids were studied in relation to the organic matter (dissolved or particulate). Natural suspended solids were studied in two stratified river mouths - the Krka River estuary and the Rhone River delta. The same studies were performed with model solids, well defined by chemical composition and preparation (controlled pore glass, silanized controlled pore glass and calcite) after exposure to natural water samples from the Krka River estuary. On the basis of the studied parameters, the existence of two types of organic compounds that mediate the surface properties of particulates was indicated: (1) hydrophilic, with high density of functional groups, reactive and degradable, and coinciding with 'river type organic matter, and (2) hydrophobic, less reactive and thus more persistent, and coinciding prevalently with 'marine type organic matter'. Generally, it was found that high Hw values (100-300-mu-J cm-2) reflect the influence of surface functional groups that originate primarily from the reactive organics (adsorbed or particulate). An extreme value of Hw (523-mu-J cm-2) was found at a location of high biological productivity and high anthropogenic influence.