The small-scale mixing of densely packed bulk material was modelled by Buggisch and Loffelmann in 1989, starting from a diffusion approach which had been developed previously. An equation could be deduced from which the diffusion coefficient appears to be proportional to the shear rate. This model conception was confirmed through shearing experiments using a Couette apparatus. The model bulk consisted of vertically packed metal rods of various diameters, parts of which were marked by colours. The present paper investigates the influence of the particle size distribution upon the mixing process. It is to be expected that the distribution of the particle size has an influence upon the package structure of the bulk and hence, amongst other things, upon the shear rate, and, eventually, upon the intensity of the diffusion. It has been possible to confirm this prediction through shearing experiments with the model bulk in the Couette apparatus.