For the optimum design and operation of gas-liquid-solid three-phase reactors, the degree of dispersion of the solid (catalyst) in the re actor must be measured, understood, and controlled. Accurate means of diagnosing the solids holdup are needed. An ultrasonic transmission technique has been developed to measure this parameter in a gas-liquid-solid bubble column reactor. The results presented in this study show that the transit time of an ultrasonic signal is influenced by the variation of solids holdup and the operating conditions in the bubble column.