To demonstrate the power of precise isotope ratio measurements of Hg in environmental samples and, more particularly, to test the use of stable isotopes as distinct tracers of the contamination source, we investigated a well-documented system, the Hg mining region near Idrija, Slovenia. Sediments alongside the Idrijca River, the Soca/Isonzo River, and in the Gulf of Trieste were analyzed to determine the variation in Hg isotopic composition versus distance from the source. Similar Hg isotopic signatures were observed among samples collected from the rivers Idrijca, Soca/Isonzo, and around the river mouth in the Gulf of Trieste, suggesting that sediments throughout the watershed of the Soca/Isonzo River to the Gulf of Trieste are dominated by Hg exported from the headwaters of the Idrijca River. Only locations on the southern part of the gulf, outside the river plume, showed lower values of the isotopic composition comparable to the Hg isotopic signature of Adriatic Sea sediments. Using a simple binary mixing-model, we could demonstrate that all samples investigated in this study were a result of variable proportions of Hg originating from the Idrija region (progressively decreasing from >90% in the northern partto <50% in the southern gulf) and from the Adriatic Sea. These results are, so far, the first evidence that tracking of mercury sources in natural systems using mercury stable isotope ratios is feasible.