In the present work, experimental investigations have been carried out on ejectors employing air as a motive fluid and water as the entrained fluid. A semi-empirical model has been developed to predict the liquid entrainment rate taking into account: (i) the compressible nature of air, (ii) pressure drop for two-phase flow and (iii) losses due to changes in cross sectional area. The effects of gas velocity, liquid level in the suction chamber, nozzle diameter and throat diameter on the liquid entrainment, entrainment ratio (L/G), pressure drop, gas hold-up, mass transfer coefficient and interfacial area have been investigated. The liquid entrainment rate increases with the increased liquid level in the suction chamber and with the increase in gas velocity. The ratio of throat cross sectional area to the nozzle cross sectional area (area ratio) was found to be a critical parameter. These results have been explained on the basis of pressure profiles of ejector (along the centre line of the ejector). The liquid entrainment rate predicted from the semi-empirical model is in good agreement with the experimental values. The mass transfer coefficient and interfacial area increase with increase in gas velocity. Correlations have been proposed to estimate the fractional gas hold-up, mass transfer coefficient and interfacial area in the ejectors. (c) 2007 Elsevier B.V. All rights reserved.