  It is shown that in isothermal isobaric diffusion of a binary fluid mixture
the mean volume velocity vanishes in the linear regime, independent of the
equation of state. The partial specific volumes of the two components are
uniform and constant in the process of mutual diffusion. The properties lead to
a simple derivation of the de Groot-Mazur thermodynamic factor in the diffusion
coefficient. The properties also imply that the diffusive volume flux defined
by Brenner is proportional to the mass current density, and is therefore not a
quantity of independent interest.
