The dependency of the main parameters of an airlift reactor on the only externally adjustable factor, the gas flow rate, has been investigated both experimentally and theoretically for two-phase (gas-liquid) and three-phase (gas-liquid-solid) systems. Experiments were performed on external airlift arrangements employing ambient air, tap water and, for the three-phase case, glass spheres. The pressure drop balance has been the basis for the theoretical predictions. The simple rig and the low gas hold-ups utilised in this work have permitted an accurate prediction of friction losses (both concentrated and distributed) based exclusively on well established relationships published in the literature, without resort to any fitting parameters; this has resulted in estimates in very good agreement with measured values. (c) 2005 Elsevier B.V. All rights reserved.