In a nonisothermal multicomponent system, there is a transport of matter by temperature gradients : this phenomenon is called thermal diffusion. Based on the thermodynamics of irreversible processes, this work : provides a rigorous background to the study of the influence of gravity on the compositional variations with depth in hydrocarbon reservoirs, proves that the theories of Haase (1969) and Kempers (1989) on "thermal diffusion" lie in the ideal case of "static thermal diffusion" : the coupling (thermodiffusion) coefficients are here equal to equilibrium quantities such as enthalpy or entropy, states that in reality, the coupling coefficients do not reduce to pure thermostatic quantities : they are related to the microscopic motion of individual molecules, so that a kinetic theory is needed, shows that, in a porous medium, the coupling coefficients should also depend on the characteristics of the medium.