  It is commonly admitted that in liquids the thermal diffusion and Dufour
coefficients $D_{T}$\ and $D_{F}$ satisfy Onsager's reciprocity. From their
relation to the cross-coefficients of the phenomenological equations, we are
led to the conclusion that this is not the case in general. As illustrative and
physically relevant examples, we discuss micellar solutions and colloidal
suspensions, where $D_{T}$ arises from chemical reactions or viscous effects
but is not related to the Dufour coefficient $D_{F}$. The situation is less
clear for binary molecular mixtures; available experimental and simulation data
do not settle the question whether $D_{T}$\ and $D_{F}$ are reciprocal
coefficients.
