Experiments were carried out in a laboratory-scale three-phase fluidized bed, with liquid as the continuous phase, with the objective of studying particle size distribution along the bed height when solid particles with a wide size distribution are fluidized. The solid phase consists of a silica catalyst support, composed of particles with sizes in the range from 25 to 404 mum. Concentration profiles of particles of each size class were measured under different conditions of liquid and gas velocities, by taking samples of the suspension at different axial and radial positions. The results extend previous observations reported in the literature, by showing that, depending on the operating conditions, particles of different size ranges concentrate at different heights along the bed, in more or less disperse profiles. Thus, depending on the liquid and gas velocities, and on the particle size, distinct patterns for the concentration profiles could be identified: segregation of small particles at the upper layer of the bed, concentration maxima of particles of different sizes at different heights along the bed, constant concentration up to a given height, followed by a decrease, and decreasing concentration along the whole bed. Particle concentrations tend to constant values as the gas velocity increases, due to particle mixing, which is promoted by the flow of gas bubbles. (C) 2004 Elsevier B.V. All rights reserved.