The local hydrodynamic behaviors, including the gas holdup, bubble size and bubble rise velocity were measured in an external loop airlift reactor (EL-ALR) with two types of distributor-porous sinter plate and perforated plate, using an optic fiber probe. The radial and axial evolutions of these parameters and the influence of the gas distributor on the flow hydrodynamics were studied. Core-peaking and wall-peaking radial profiles of the gas holdup were found in the experiments corresponding to different bubble sizes. A mechanism model based on the assumption of the equilibrium of lateral forces acting on a bubble was proposed to interpret the different radial profiles of the gas holdup. (C) 2004 Elsevier B.V. All rights reserved.