During chemical weathering, magnesium (Mg) is released by the dissolution of both carbonate and silicate sources. The degree to which solute concentrations and isotopic compositions of rivers reflect the relative proportions of these two inputs, or cycling by a series of processes associated with weathering is poorly constrained. In the river waters of the Mackenzie Basin (Canada), the Mg content is high and Mg isotope ratios (26Mg/24Mg expressed as View the MathML source) show in excess of one per mil variability. Part of this variability is attributed to the 3‰ range in the carbonate and silicate rocks drained. Despite this inherent lithological control on river water View the MathML source values, there is also evidence for a fractionation control. A linear positive covariation between lithium (7Li/6Li, expressed as View the MathML source) and Mg isotope ratios in the river waters of the Mackenzie Basin is reported. This covariation is not expected because previously reported fractionation related to physicochemical processes associated with clays or oxides should induce a negative covariation with Mg isotope ratios.