Electrical conductivity measurements in solid-liquid and solid-liquid-gas dispersions have proven suitable for determination of the volumetric fraction of the phases, or hold-up. A combination of estimation routines was applied to determine the gas hold-up and solids hold-up on line in a flotation column. Gas holdup was determined by using the so-called 'phase separation method' and the standard addition method was used to estimate solids hold-up. Laboratory results validating the techniques and results obtained from applications in industrial flotation columns are presented.