Application of nonequilibrium thermodynamics to study thermodynamic properties of non-ideal double mixtures and their relationship with transfer coefficients is considered. A number of general relationships used to calculate thermodynamic mixing potentials have been obtained. Thermal diffusion has been shown to exist only in ideal systems. Theoretically and experimentally, the diffusion thermal effect was analyzed and it was proved that this is a local manifestation of the heat of displacement in the process of mutual diffusion of the starting materials.