A technique based on electrical conductivity to measure holdup in three-phase reactors has been developed. The technique relies on measurement of the effective conductivity of the dispersion and of the continuous phase alone, and on analysis of the data using available models describing the effective conductivity of dispersions. A conductivity cell, consisting of two "grid" electrodes covering the cross-sectional area of the reactor (column) and separated by a set distance, was used to perform the measurements of conductivity. Such a cell is an adaptation of the "ideal" cell consisting of two infinite and parallel plates where absolutely uniform potential and current (electrical) fields exist. By using this cell, the free movement of the phases is allowed and conditions for near-to-uniform electrical fields are met, thus allowing sound measurements of the effective conductivity of the dispersion to be made. The technique was first validated in water-air (0-25% v/v air) and in water-mineral (0-20% v/v solid) systems. Maxwell’s model (1892) adequately described the experimental data. Results in a three-phase system showed that holdup estimates from Maxwell’s model were also in good agreement with those obtained by direct measurements.