During mixing of river water and seawater, changes in water chemistry can affect the behaviour of elements in estuaries, and consequently may alter their input into the oceans. In order to evaluate this, lithium (Li) concentrations and isotope compositions of estuarine waters from the Kalix and Råne River estuaries (Northern Sweden) have been determined. The waters span δ7Li values of +21.7 to +30.4 ‰, with the lowest values found within the shallowest samples with the lowest salinities. Dissolved Li concentrations show near conservative behaviour with increasing salinity; however their δ7Li isotopic compositions may be explained by either conservative mixing with an intermediate salinity Bothnian Bay end member; or non-conservative behaviour, with excess Li added to solution during mixing. The non-conservative behaviour of Li might be ascribed to cation exchange processes with suspended riverine particles or weathering during mixing with seawater within the low salinity zones of the Kalix and Råne River estuaries.The existence of non-conservative mixing of Li in estuarine environments suggests that the Li isotopic riverine input signature could be modified within estuaries, which may affect the marine and palaeo-marine budgets. Hence, this has important implications for the estimation of the fluvial Li flux into the oceans