Phase holdups were measured in the riser section of a gas-liquid-solid circulating fluidized bed (GLSCFB). Electrical resistance tomography (ERT) as a non-invasive imaging technique, pressure transducers (PTs) and fibre optic probes were employed. Water was used as continuous and conductive phase, air as the gas phase and glass beads as solid nonconductive phases. ERT technique is based on conductivity measurement of the continuous phase (water in this study), which provides color-coded cross-sectional view of phases with a frequency of up to 250 images per second. The local conductivity measured by a number of electrodes located at the periphery of the plane was then further converted into a local phase concentration distribution based on Maxwell's relation. The results obtained by PTs, when combined with ERT results, were used to determine gas and solid holdups. Fibre optic probe was also employed to measure gas holdup independently. To measure gas and solid holdup, a model was introduced to exploit the fibre optic data in differentiating gas bubbles from solid particles in the riser. Radial profiles of the phase holdups were determined. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.