Research was conducted in various experimental stations throughout Gulf of Trieste and Grado and Marano Lagoon. Ecosystems observed are governed by different physical, chemical and biological conditions. The main aim of this research was to investigate the existence and extent of interactions between organic matter (OM) and mercury (Hg). The influences of various environmental conditions were also studied. The most important factor in our study was the effect that the origin, thus structure and composition, of OM as well as Hg has on the existence and extent of interactions. Gulf of Trieste is contaminated with Hg from Idrija mercury mine whereas Hg found in Lagoon has two origins. One is Idrija mercury mine, while the other is chloralkali plant operating until recently and situated upstream of Aussa River. Aussa River is as an Aussa-Corno river system discharged into the Lagoon. During the researchdissolved and sedimentary OM and its interactions with Hg were investigated. It turned out that dissolved OM present in water column plays animportant role as Hg and other metal complexation agent, however no conclusive evidence was found that interactions between sedimentary OM and Hg exist. Similar results were obtained also on samples from the lagoon environment. It was concluded that the absence of interactions in sediments was a consequence of OM quality, meaning that its recalcitrance does not support Hg binding in the extents as water column OM does. Water column OM is apparently more bioavailable and contains functional groups more suitable for metal binding. Macroaggregates, large aggregates of OM, are relatively regularly formed in the waters of northern Adriatic, also in waters of Gulf ofTrieste. Knowledge on mechanisms of macroaggregate formation and degradationis necessary, since it offers an invaluable insight into dynamics of natural OM as well. Analyses on macroaggregate matrix showed high metal concentrations which suggested an incorporation of inorganic particles, while the results on interstitial soluble portion of macroaggregates revealed similar to identical properties in terms of structure and metal binding capacity as natural OM. In future the combined usage of ultrafiltration and radioactive or stable isotopes could prove to be invaluable tool for identifying the mechanisms behind interactions between OM and Hg. OM is one ofthe most important actors in the biogeochemical cycle of Hg, since it servesas a fuel for microbial metabolism. It supports also the microbes responsible for transformations of Hg. The experiments performed in different environments where OM and Hg have different origins allowed us to confirm the hypothesis on importance of bioavailability of OM and Hg in the formation of MeHg.