This paper discusses two series of experiments performed in a shear cell device with six different amounts of silicone oils and using 2-mm soda lime beads as the granular materials. The first series of experiments were mixing experiments, and the developments of mixing layer thicknesses were measured. The second series of experiments had the same experimental conditions as the first series but used different combinations of colored particles. In the second series of experiments, the motions of granular materials were recorded by a high-speed camera. Using the image processing technology and particle tracking method, the positions and velocities of particles were measured. The self-diffusion coefficient could be found from the history of the particle displacements. The measured mixing layer thicknesses are compared with the calculations from a simple diffusion equation using the data of apparent self-diffusion coefficients obtained from the current measurements. The comparisons show good agreements, demonstrating that the mixing process of granular materials occur through the diffusion mechanism in this cohesive sheared flow. In addition, the apparent self-diffusion coefficient decreases with the increasing liquid volume, indicating that cohesive effect between particles reduces granular mixing. (c) 2007 Elsevier Ltd. All rights reserved.