Ultrasonic technique has been recently developed to measure dispersed phase holdups in multiphase flows. The experimental results obtained in this work have shown that the fluctuations of the sound speed and attenuation of ultrasound are well-defined functions with solid and gas holdups in liquid-solid and gas-liquid two-phase flows. When both solid particles and gas bubbles are present in a liquid flow, gas bubbles appear to be a dominant factor for the instability of ultrasound signals. These findings lead to a new approach for simultaneous detection of gas and solid holdups in a gas-liquid-solid three-phase flow. In this work, canonical analysis and optimization methods are successfully applied to differentiate the contributions of gas bubbles and solid particles to the ultrasonic signals recorded from three-phase systems. The gas and solid holdups determined by the proposed approach are in a good agreement with the experimental results obtained using differential pressure transducers and conductivity probes. (C) 2004 Elsevier Ltd. All rights reserved.