Diffusion between single-crystal films which are in contact and in parallel orientation is sometimes accompanied by the generation of grain boundaries. The grains enclosed by these boundaries are unusual in that their lattices are curved to accommodate the difference between the lattice parameters of the upper and lower specimen surfaces. In the regions outside these boundaries the gradient in lattice parameter is accommodated by misfit dislocations. The grain boundaries themselves are made up of dislocations which once accommodated misfit in the areas occupied by the curved grains. The formation of curved grains depends upon the misfit between the two crystals, and upon their relative thickness.