The solid state partitioning of metals and As in river channel sediments has been investigated along an 828 km study reach comprising the rivers Lapuş, Someş and Tisa, using a 4-stage sequential extraction procedure (SEP) that identified 4 chemical phases: (1) exchangeable, (2) Fe and Mn oxides, (3) organic matter/sulphides and (4) residual. Peak total concentrations of As (240 mg kg−1), Cd (95 mg kg−1), Cu (24,500 mg kg−1) and Pb (21,100 mg kg−1) were found in the River Lapuş immediately downstream of Băiuţ base-metal mine, with Zn concentrations (23,000 mg kg−1) peaking 2 km further downstream. Contaminant metals and As within samples close to Băiuţ mine were largely bound to sulphides (>92%). Ten km downstream from Băiuţ mine, however, Cd and Zn were found to be environmentally significant due to their strong association with the exchangeable phase (73 and 47%, respectively). Lead was found to be predominantly associated with Fe and Mn oxides (71%), but concentrations in the residual phase were relatively high in the upper Tisa. Copper was more evenly distributed between the 4 phases than the other 4 elements analysed. Elevated metal and As concentrations were found in the rivers Lapuş and Someş downstream of polluted tributaries, but sites on the River Tisa had total metal concentrations below Dutch guideline values. The implications of the river sediment metal partitioning results on the environment are considered in light of Cd and Zn ‘exchangeable hotspots’ at sites on the Lapuş and Someş Rivers.