Scale-up criteria for obtaining the same vertical concentration profiles in agitated suspensions are discussed. The experiments were carried out in reactors of two scales (V = 39.6 and 261 1) characterised by a high aspect ratio and stirred with multiple, evenly spaced impellers of two different types. The profiles were determined under different conditions: at constant tip speed, at constant specific power consumption, and at an intermediate condition (i.e., N proportional to D-0.93). The experimental profiles were compared with different approaches, namely on a qualitative basis, in terms of standard deviation and the parameter of the axial dispersion model with sedimentation. In all cases, the same criterion based on the aforementioned intermediate condition (closer to constant tip speed) was confirmed. The experimental data were also examined in terms of effective particle settling velocity, which is a basic parameter for modelling, and fair agreement of the data obtained at the two scales with the different impellers was obtained. (C) 2003 Elsevier Ltd. All rights reserved.