The monolith bed is one of the promising catalytic reactors for a number of chemical gas-liquid-solid processes. In the present work, liquid saturations for five different monoliths have been investigated experimentally in a cold-flow unit with a reactor diameter of 5.0 cm. The influences of gas and liquid flow rates and of the direction of two-phase flow on liquid saturation were examined. The results indicate that the direction of flow has no significant influence on liquid saturation for proper gas-liquid distribution. The experimental results are in good agreement with predictions of the drift flux model using the distribution parameter proposed by Ishii (ANL Report ANL-77-47, 1977) along with the assumption of zero drift velocity. In preliminary experiments, gamma-ray computed tomography (CT) has been successfully applied to measure time-averaged liquid distribution over the monolith cross-section in a selected condition. The employment of a nozzle-type distributor provides an almost uniform liquid distribution over the monolith substrate. It is demonstrated that CT is a viable technique for studying two-phase flow in laboratory-scale monolith reactors. (c) 2005 Elsevier Ltd. All rights reserved.