Flotation is a separation process in which a hydrophobic material is separated from a hydrophilic one. It is commonly used in several branches of processing. The purpose of this paper was to use three dimensional Electrical Resistance Tomography (ERT) for the measurement of the spatial distribution of gas holdup in a mechanical flotation cell. Using an array of metal electrodes installed on the wall of the cell, a set of electric currents was injected to the cell and the resulting voltages were measured. The electrical conductivity within the cell was estimated based on the known currents and measured voltages. The gas holdup distribution was computed based on Maxwell model. The method was tested in a 50 dm(3) mechanical flotation cell filled with water. The gas holdup distribution is presented at various values of the rotor speed and gas superficial velocity in non-frothed and frothed cases. Moreover, the performance of two rotor-stator mechanisms was compared. The results indicate that the 3D gas holdup distribution in a mechanical flotation cell under different conditions can be estimated using ERT. Moreover, differences in the gas holdup distribution can be detected depending on which rotor-stator mechanism is in use. (C) 2011 Elsevier Ltd. All rights reserved.