The hydrodynamics of two configurations of internal airlift reactors, both with a riser diameter of 0.1 m, operating with an air-water system, have been experimentally investigated for a range of superficial gas velocities. The experimental results are compared with a model using Computational fluid dynamics (CFD) with Eulerian descriptions of the gas and liquid phases. Interactions between the bubbles and the liquid are taken into account by means of a momentum exchange, or drag, coefficient based on a literature correlation. The turbulence in the liquid phase is described using the k-epsilon model. The CFD model shows excellent agreement with the measured data on gas holdup, liquid velocity in the downcomer and in the riser. The developed CFD model has the potential of being applied as a tool for scaling up. (C) 2002 Elsevier Science B.V. All rights reserved.