The hydrodynamics of fluid cokers were studied in a pressurized fully cylindrical cold model scaled by matching key dimensionless groups so that the reactor sections of the commercial and laboratory units were geometrically and dynamically similar. Voidage distributions and solids flow structure were investigated using optical fibre probes in a fluidized bed of FCC. The results show a relatively dense annular region, in which the time-average flow is downwards, surrounding a more dilute, upward flowing core region. However, the radial profiles of voidage indicate that the voidage increases gradually from the wall to the axis of the column, rather than showing a sharp transition between an annular and core region. Particles in the descending outer region are entrained by the feed jets into the core region. The radial position of the boundary between upward and downward net solids flow did not change significantly when the superficial gas velocity was varied. (C) 2004 Elsevier B.V. All rights reserved.