The misfit between one film and another is often accommodated by misfit dislocations. If the crystals are miscible and are allowed to interdiffuse the misfit dislocations become distributed in the alloyed volume. Frequently, some parts of one dislocation move away from the interface into one crystal and other parts move into the other crystal. The parts in one crystal are connected to those in the other by dislocations that thread the diffusion zone. The density of these threading dislocations depends on the misfit between the two crystals, and may be influenced by the Kirkendall effect and by the misfit accommodated by elastic strain. Interaction between misfit dislocations in the diffusion zone often leads to the creation of new grains. These grains are unusual in that their lattices are curved to accommodate misfit between the upper and lower film surfaces. The boundaries to the grains are approximately perpendicular to the original interface and are made up of misfit dislocations that once occupied the material inside the curved grain. As dislocations and grain boundaries enhance diffusion the threading dislocations, and the boundaries to the curved grains, are expected to contribute to mixing of the films. © 1975.