Experiments conducted quantify the hydrodynamic characteristics of a two-dimensional (2D) intemal-loop airlift reactor in a gas entrainment regime, which is divided into two sub-regimes differentiated by re-circulating bubbles back to the riser (i.e., Regimes I and II). Both flow visualization and full-field measurements using particle image analyzer (PIA) have been performed. Contours for instantaneous vorticity, averaged velocity, and Reynolds stresses are obtained and discussed in relation to measurable coherent structures present in the flow. In Regime I, the riser comprises four coherent structures, including the descending, vortical, fast bubble and central plume flows. Due to reducing the local density by trapping small bubbles in the vortical flow along with recycling a small portion of liquid back to the riser, the flow conditions exhibit a weaker vortical flow, less significant changes in both normal and shear stresses, and the inversion point shifts to a location 90% diameter of the riser. For Regime II, the central fast bubble flow results in a parabolic profile of the vertical velocity with a maximal value at center and an inverse point at 70% diameter of the riser. The maximal values of the normal stresses, &LANGBRAC; U'U'&RANGBRAC; and &LANGBRAC; V'V'&RANGBRAC;, are located at the centre of the riser and are induced by the swing of the fast bubble flow and close to the sidewall provoked by the vertical fluctuation of the vortical flow, respectively. © 2005 Elsevier B.V. All rights reserved.