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Mercury is one of the most harmful elements present in the Earth, and has both natural and anthropological sources. Moreover, Hg can undergo to many different transformation pathways during its biogeochemical, or industrial, cycles which in general involve redox reaction, both abiotic and biotic, and phase changes (1). Despite the toxicity of this pollutant, there is still a lack in the knowledge about the biogeochemistry of mercury in the ecosystem and, therefore, it is of utmost relevance to develop new scientific approaches to understand its transformation mechanisms and to identify its contamination sources. In this context, the determination of mercury stable isotopes ratios and, in particular, the identification of fractionation processes seems to be an extremely interesting and challenging application to verify the “provenance” of the element. Mercury, in fact, undergoes to both mass dependant, MDF, and mass independent fractionation, MIF, processes. In particular the MIF, involving only the odd isotopes (199Hg and 201Hg), appears to be a characteristic fingerprint of the process and the pathways involved in the Hg transformations (2). Thus, the study of both fractionation phenomena can be a powerful tool to identify its natural or anthropogenic source. This approach can be useful in case of polluted areas where many are the contamination sources in order to plan an environmental requalification. An intriguing case study is represented from the National Interest Site of the lagoon of Marano-Grado (Trieste, Italy), which is object of the present study. For these purposes, the evaluation of the isotopic composition in samples coming from this area …