A mathematically transparent model for long-term solute dynamics, based on an oscillating reference frame, is applied to the analysis of the mixing process in estuaries. Classical tidally-averaged transport models for estuaries, all derived in some way from the Fractional Freshwater Method of Ketchum (1951) are reinterpreted in this framework. We demonstrate that in these models, the dispersion coefficients obtained from salinity profiles are not always a good representation of the mixing intensity of other dissolved constituents. In contrast, the hypothesis of equal coefficients is always verified in our oscillating coordinate system, which is almost devoid of tidal harmonics. The math- ematical representation of the seaward boundary condition is also investigated. In the tidally-averaged Eulerian models, a fixed Dirichlet boundary condition is usually imposed, a condition that corresponds to an immediate, infinite dilution of the dissolved constituent beyond the fixed estuarine mouth. This mathematical representation of the estuarine-coastal zone interface at a fixed location is compared with the case of an oscillating location, which protrudes back and forth into the sea with the tide. Results demonstrate that the mathematical representation of the seaward boundary condition has a significant influence on the resulting mixing curves. We also show how to apply our approach to the prediction of mixing curves in real estuaries.