In the case of a mixture of more than two constituents it is known that the diffusion of a given constituent depends not only on the concentration gradient of that constituent but also upon the gradients of the other constituents. This is known as diffusion-drag effect, and the theory postulates the diffusion of constituent k is governed by
,
in which there are K - 1 values of the coefficients
for constituent k, and K - 1 such sets of
coefficients. The coefficients 
are closely
related to the usual diffusion coefficients, while the
coefficients 
characterize the
diffusion-drag phenomenon.
The second law predicts that the values of the coefficients characterizing diffusion-drag effect are controlled to a certain extent by the values of the diffusion coefficients. In order to demonstrate this, consider the case of a mixture of ideal gases for which
In the absence of temperature gradients, the diffusion fluxes of the gas constituents take the form
where
The second law requires that
for a uniform temperature field. Using Eq. (5.04-1), this becomes
By similar reasoning as in the preceding section, it can be shown ( ()) that if (5.04-6) is to be satisfied for all values of the concentration gradients, then we must require that
and
The requirement on the coefficients of Eq. (5.04-3) are therefore
and
