The hydrodynamics of gas-liquid cocurrent down- and upflow packed beds was studied experimentally using twin-plane electrical capacitance tomography (ECT) The ability of ECT to measure liquid holdups was examined by performing different calibration procedures Confrontation of liquid holdup measurements from single-/twin-plane ECT RTD and bed drainage revealed that ECT calibration between pre-wetted and flooded bed captured accurately the free-draining liquid holdups A new modality of imaging tracer transients referred to as twin-plane ECT/RTD was proposed whereby upstream injections and detection of permittivity-contrasting liquid tracer impulsions allowed access via voxel-affixed RTD and cross-con elation analyses to interstitial velocity liquid holdup and volumetric flux maps as well as to their degrees of uniformity Single-plane ECT was successful at monitoring the liquid drainage dynamics showing its dependence with respect to gas flow rates and particle sizes as well as to identify regime transitions and to unveil the relationships of pulse frequency and pulse velocity with respect to particle diameter and fluid throughputs in upflow and downflow (C) 2010 Elsevier B V All rights reserved