Over the last six years, we have published a series of papers developing a hydrodynamic model of thermodi¤usion in liquids and liquid mixtures. Initial work focused on the movement of polymers dissolved in nonpolar solvents arising from the pressure gradient induced by asymmetry in polymer–solvent interactions as a result of the temperature-induced concentration gradient in the solvent. As that model was refined, we examined secondary e¤ects of a macroscopic pressure gradient in the solvent, and finally extended our hydrodynamic approach to cross-di¤usion and thermodi¤usion in solvent mixtures. Application of the model is currently limited to non-ionic liquids, where molecular interaction energies can be estimated by Hamaker constants or similar parameters. In this paper we present the body of work as a whole, in order to ensure a consistent nomenclature and unify the model for application to liquid mixtures in general. To test the model, values of Soret coe‰cients for mixtures of toluene and n-hexane have been calculated using parameters in the literature.