The interaction between viscous stress and diffusion is examined through the use of the Chapman and Cowling method, and the results for a general binary mixture of monatomic gases is obtained. It is demonstrated that the kinetic theory calculation provides a first iteration to the form predicted by statistical mechanics; it also produces a modification in the barodiffusion coefficient that is in more general agreement with the results of the 13‐moment method than has been shown previously. Finally, the general equations are modified to permit description of a dusty gas or aerosol of fine particles, and the result is found to differ significantly from a calculation based on the methods of stochastic processes.