Laboratory experiments have been performed on the flow of oil, water and air through a vertical pipe in order to study the gas-lift technique for oil-water flows. Special attention was paid to the phase inversion phenomenon, by which the continuous phase switches to the dispersed phase and vice versa. By using different types of gas injectors the influence of the bubble size of the injected air on the efficiency of the gas-lift technique (in particular at the point of phase inversion) was studied. Also the gas and liquid mixture velocities were varied. The air bubbles were detected by means of optical fibre probes. Local measurements of the time-averaged gas volume fraction, bubble size and bubble velocity were carried out, as well as pressure measurements. Phase inversion is associated with a sharp increase of the pressure gradient. With air injection the pressure gradient (for the oil-water-air flow) is always significantly smaller than for the case of oil-water flow, except at the point of phase inversion where the pressure drop can be even be higher than for oil-water flow. It was also found that air injection does not significantly change the critical concentration of oil and water where phase inversion occurs. An important phenomenon to explain these results is, that the presence of the dispersed (oil or water) phase has a significant influence on the bubble size. Also the influence of the gas flow rate on the distribution of the oil and water phases in a cross-section of the pipe plays a significant role. (c) 2007 Elsevier Ltd. All rights reserved.