This paper reports some observations on the behaviour of a segregated particle bed consisting of a mixture of small and large particles in the transverse plane of a rolling mode rotating drum. A non-invasive positron emission particle tracking (PEPT) technique was used to follow the trajectories of different sized particles. A two-dimensional mathematical model based on the Eulerian approach and the thin layer approximation was developed to simulate solids motion and concentration distribution of different sized particles. It is shown that the bed structure for a binary mixture is similar to that for mono-sized particles, i.e. two-region structure consisting of a relatively thin 'active' region and a 'passive' region near the drum wall where particles move as a rigid body. At low rotational speeds, the velocity difference between small and large particles of a binary system is negligible in both the active and passive regions. At relatively high rotational speeds, the velocity difference is small in the active region and negligible in the passive region. Radial occupancy data suggest that small particles tend to concentrate in the core region, whereas large particles tend to occupy the shell region, in good agreement with model predictions. Axial occupancy data reveal that the axial particle mobility increases with drum rotational speed, and the mutual diffusivity of small particles is higher than that of large particles. It is also shown that the turnover time ratio of small to large particles is almost independent of concentrations of small particles under the conditions of this work. (C) 2002 Elsevier Science Ltd. All rights reserved.