Mercury distribution and partitioning was studied in the River Idrijca system, draining the area of the former Idrija mercury mine, Slovenia. Mercury dynamics were assessed by speciation analysis of mercury in water and river bed sediment samples during a 2-year study at locations on the River Idrijca and its major tributaries. Simultaneously, the influence of some major physico-chemical parameters that influence the fate of mercury in the aquatic environment was investigated. The distribution of mercury species in the River Idrijca catchment indicated contamination from mine tailings distributed in the town of Idrija and erosion of contaminated soils. The partitioning between dissolved and particulate mercury phases in river water was found to be mostly controlled by the variable content of suspended solids resulting from changing hydrological conditions and complexation with various ligands present in river water, among which dissolved organic carbon (DOC) seems to be the most important. Overall results indicate that mercury is transported downstream from the mining area mainly as finely suspended material including colloids rather than in the dissolved phase. This riverine transport occurs mostly during short, but extreme hydro-meteorological conditions when remobilization of mercury from the river bed sediments occurs. A significant part of the mercury particulate phase in water corresponds to cinnabar particles. During its transport, important Hg transformation mechanisms that increase the risk of mercury uptake by biota take place, evidenced by the increase in the relative contribution of reactive mercury (Hg R ), dissolved gaseous mercury (DGM) and monomethylmercury (MeHg) downstream from the Idrija mine. However, our data revealed relatively low methylation efficiency in this contaminated river system. We attribute this to the site specific physico-chemical conditions responsible for making inorganic mercury unavailable and limiting the capacity of methylating bacteria.