Abstract Most biogeochemical processes in natural waters take place at different phase discontinuities, among which the largest one is the atmosphere– ocean boundary. The sea surface microlayer is a thin layer with special physical, chemical and biological properties. Organic films and surface tension forces provide physical stability of the surface microlayer, where substances, suspended particles and organisms can be concentrated. Surface properties are important because they affect several macroscopic properties such as wave damping, microwave backscatter, gas transfer, wave breaking and bubble aerosol formation. High surface tension of the air– water interface energetically favours adsorption of amphiphilic organic compounds. Owing to molecular properties, surface-active chemical species are adsorbed at the air– water interface, i.e. their concentration increases spontaneously. Marine surfactants are primarily produced by phytoplankton and show seasonal variability. 2 B. ´Cosovi´c The sea– air interface is a collector of marine anthropogenic pollutants. Surfactants mediate long-range transport of pollutants by marine aerosols. The Mediterranean Sea is a semienclosed sea, which is characterized by a high anthropogenic pressure and high solar radiation and thus it is expected that processes at the sea surface play an important role in the control of biogeochemistry of materials in the Mediterranean Sea.