During continuous flow of solid-liquid mixtures, the solids may be oriented in a variety of directions relative to the flow axis. The character of the distribution takes on particular importance when materials are processed using electric fields. In this study, solids orientation distributions in continuous, vertical upflow was investigated experimentally combining particle tracking velocimetry. Cylindrical solids showed a more pronounced tendency than cubes to align along the flow direction; this tendency was even greater for elongated cylindrical objects. Solids orientation is influenced by the principal orienting factor (shear flow) and the randomizing factor (orientations of other solids in the mixture). Increasing flow rate tends to drag objects towards their equilibrium (aligned orientation) until they are restricted from further rotation by the presence of other solids. Orientations of cubes were more widely distributed than cylinders, without sharp peaks. The orienting effects of flow rate were also observed for cubes. High solids concentration and larger solids tend to restrict rotation, thus the initial distribution may be maintained through the flow length. (C) 2004 Elsevier Ltd. All rights reserved.