Various studies on ion-beam mixing suggest that the extent of mixing is sensitive to the sign and magnitude of the heat of mixing, DELTAH(m). This implies a role, not only for random motion, but also for chemical driving forces of the type where the total diffusion flux is modified by the factor [1 - alpha(i)(1 - alpha(i)) 2h(m)p/RT(1 + p)]. Here alpha1 is the atomic fraction of component i, alpha(i)(1 - alpha(i))h(m) is the heat of mixing of a regular solution, and p is the ratio of the diffusivities for chemically guided defect motion to those for random motion of all types. The parameter p has never been evaluated for any system and we wish to evaluate it first by analyzing the profiling experiments of Marton, Fine, and Chambers on multilayers of Ni-Ag. We then obtain further values of p from ion-beam mixing experiments on bilayers and multilayers. It is shown that it is possible to understand a variety of experimental results relating to profiling and to ion-beam mixing in terms of chemical driving forces and, moreover, to do so without invoking thermal spikes.