Mixing experiments were performed in a shear cell device using differently colored but otherwise identical glass spheres, with five different bottom wall velocities. The motions of the granular materials were recorded by a high-speed camera, so the development of mixing boundaries could be analyzed from the images. By continually tracking the movements of particles, the velocity, fluctuations and the granular temperatures were measured. The self-diffusion coefficients were determined from the histories of particles movements. The mixing layer thicknesses were compared with the calculations from a simple diffusion equation using the data of self-diffusion coefficients obtained from the current measurements. The calculations and experimental results showed good agreements, demonstrating that the mixing process of granular materials occurred through the diffusion mechanism. (c) 2005 Elsevier Ltd. All rights reserved.