The present experimental research investigates transverse particle diffusion in a flow of granular material. The mixing-layer experiments are conducted in a vertical channel fed by an entrance hopper that is divided by a splitter plate. On either side of the splitter plate differently colored, but otherwise identical glass beads, enter the test section. Downstream of the splitter plate the colored particles collide which may result in a migration of particles across the centerline. The measurements include the distribution in color concentration across the channel, and the distributions in the local velocity and fluctuating velocity in the flow direction. The results indicate that the diffusion rate is influenced by the flow rate, and by the wall shear which depends on the surface conditions of the channel side walls and the channel width.