Specialty designed internals were used in a novel multi-stage internal-loop airlift slurry reactor to decrease liquid backmixing and provide for a homogeneous suspension of solid particles. There was three bubble circulation regimes with different bubbling characteristics in the downcomer of the bottom stage: bubble-free regime (BFR), transition regime (TR) and complete bubble circulation regime (CBCR). The effects of the unaerated liquid level and superficial gas velocity on the bubble circulation regimes were investigated. The unaerated liquid level significantly affected the regime transition from the TR to CBCR, but had a negligible effect on the transition from the BFR to TR. The axial profile of the gas holdup in the downcomer was non-uniform in the BFR, but almost uniform in the CBCR. With increasing superficial gas velocity, the liquid circulation velocity increased in the BFR, remained approximately constant in the TR, and then increased in the CBCR. A mechanism for the regime transitions that was based on the measured gas holdup, liquid circulation velocity and bubble penetration depth was given. (C) 2008 Elsevier B.V. All rights reserved.