Acoustic velocity measurements have been used to estimate particle size and concentration in liquid-solid (LS) and gas-liquid-solid (GLS) systems. The slurries of 34 and 110 mu m particles and their binary mixtures in distilled water are tested in concentration range 2-10vol% in batch operations. In GLS systems, the superficial gas velocity is varied from 0.03 to 0.10 m/s. The presence of gas bubbles affected the acoustic velocity measurements significantly. A data filtration procedure is developed to minimize the effect of gas phase and estimate variations in gas-free liquid-solid region. The filtered data are used to predict concentration using calibration equations and theoretical velocity prediction models of LS systems. The estimated concentrations are comparable with sedimentation-dispersion model predictions in GLS system. This technique is also used to predict average particle size and composition in binary mixtures of particles. (c) 2005 Elsevier Ltd. All rights reserved.