Although the production of mercury (Hg) at the Idrija mercury mine, Slovenia, stopped in 1994, the tailings and contaminated soils in the mining region are continuously eroded and serve as a permanent source of Hg to the downstream rivers, flood plains and the Gulf of Trieste. The present article describes measurements of total Hg and monomethylmercury (MeHg) in various environmental compartments in the rivers Idrijca and So|$$|Ahca and the Gulf of Trieste during the period 1998–2000. Total Hg in the Idrijca river water upstream of the mercury mine varied between 2.8 and 6.9 ng l−1 and increased to 197 ng l−1 downstream of the mercury mine and remained elevated further downstream along the rivers Idrijca and So|$$|Ahca (from c. 10 to 59 ng l−1), with lowest concentrations found in marine waters (from 0.2 to 2.0 ng l−1). The concentrations of total Hg were higher after the rain, which caused erosion and transport of particles enriched with Hg. Concentrations of MeHg were quite variable and did not follow the same trend as total Hg. The concentrations of MeHg in August 1999 were significantly higher than in June 1998, indicating the importance of seasonal effects, as well as hydrological conditions, on the production of MeHg. Above the mercury mine, MeHg was 0.1–0.2 ng l−1 and doubled below the mercury mine. Downstream, in the rivers Idrijca and So|$$|Ahca, MeHg varied from 0.05 to 0.53 ng l−1 and in marine waters from 0.06 to 0.08 ng l−1. Concentrations of Hg in sediments also increased downstream several fold with a value of 5.39 μg g−1 above the mercury mine to a maximum of 727 μg g−1 below the mine in the silt and clay fraction. The coarser-grained fraction (0.063 < d < 1.4 mm) frequently exceeded the value of total Hg in the silt and clay fraction due to the presence of particles of cinnabar. The concentrations of MeHg in sediments did not follow the same trend as total mercury. Highest concentrations of 14.1 and 10.0 ng g−1 were found downstream of the peak total Hg concentration in sediments. In general, higher MeHg concentrations were associated with the silt and clay fraction, and accounted for 0.001–0.144% of total Hg. Total Hg in flood plain surficial soils varied from 60 to 150 μg g−1 with less than 0.01% of mercury as MeHg. The results confirmed the fact that beside soils that are naturally elevated in Hg due to the high levels of Hg in geological materials in the Idrija district, there are also Hg-laden materials and tailings that are continuously eroded and deposited in flood plains downstream and serve as an important additional source of Hg to the rivers and the Gulf of Trieste. Important transformation mechanisms take place during fluvial transport, evidenced by the presence of MeHg in the various samples analyzed.