Vessels filled with liquid mixtures under high pressure and at high temperatures are used in many chemical processes. In the case of a process-breakdown, a sudden decrease of the pressure can occur. As a result, the liquid level inside the vessel swells and a blowdown of liquid and gas from the vessel occurs. For the operation of the plant, the design of the apparatus has to be safe and has to avoid similar states of operation. Therefore, the hydrodynamic phenomena occurring inside the vessel during the blowdown have to be known. Using electrical tomography it is possible to measure the distribution of the void fraction in each cross section of the vessel during the blowdown operation. (C)2000 Elsevier Science S.A. All rights reserved.