There are two approaches to the interaction between solute atoms and migrating interfaces. A comparison between the two is illustrated with molar Gibbs energy diagrams. It is demonstrated that the present treatments of solute drag are equivalent to the treatment based on dissipation of Gibbs energy for grain boundary migration but not for phase transformations. A new treatment of solute drag is equivalent to the dissipation approach for both cases. It predicts that the solute drag changes sign for phase transformations and acts as a driving force.