The Cd concentrations and isotopic compositions were determined for 19 water samples which cover the mixing zones of four major rivers with the coastal seas of the Siberian Shelf. The waters span salinities from about 1–32, with Cd concentrations of about 0.02–0.46 nmol/kg and small but resolvable Cd isotope fractionations, with ε114/110Cd values of between +1.4 and +5.7. The data for the majority of the samples are in accord with the systematics expected for quasi-binary mixing of Arctic seawater (ε114/110Cd≈5.5±0.5 and Cd concentration between 0.1 and 0.25 nmol/kg) with river waters characterized by ε114/110Cd≈+2±1 and low pristine Cd contents of about 0.02–0.06 nmol/kg. The river values are similar to the inferred composition of the continental crust, which implies that weathering produces no or only limited Cd isotope fractionation. The results for five samples provide clear evidence for non-conservative behaviour of Cd, as the element is released from suspended riverine particles during mixing with seawater. The isotopic data furthermore show that the desorbed Cd is characterized by ε114/110Cd≈+3, in accord with a natural origin. This implies that the (natural) net riverine Cd fluxes of the Siberian rivers to seawater are also likely to be characterized by ε114/110Cd≈+2±1, a value that is either identical to or intermediate between the composition of the continental crust and marine deep waters. Additional data for the boreal Kalix River in Sweden contrasts with the results obtained for the Siberian rivers, as the former exhibits a much lighter Cd isotope composition of ε114/110Cd=–3.8 coupled with a much higher Cd content of ∼0.24 nmol/kg. These characteristics appear to be a consequence of the distinct hydro-geological setting of the Kalix drainage basin, which suggests that the riverine input of Cd isotopes to the oceans might display significant regional variability. In summary, our study underlines the important role that stable isotope analyses can play in biogeochemical investigations of trace metals. Here, the Cd isotope results provide important constraints, which are not available from concentration data alone, on the cycling of Cd in riverine and shelf environments.