AbstractThe effects of the horizontal connection length (0.1 ±Le± 0.5 m), the cross‐sectional area ratio of downcomer‐to‐riser (0.11 ± Ad/Ar± 0.53), and the superficial gas velocity on gas phase holdups in the riser and downcomer were studied. The circulation liquid velocity, the mixing performance and the volumetric mass transfer coefficient in the external‐loop airlift reactors were also measured. The horizontal connection length andAd/Arwere major parameters which strongly affected the performance of external‐loop airlift reactors. Useful correlations in the external‐loop airlift reactors were obtained for gas holdups, the volumetric mass transfer coefficient, the circulation liquid velocity, and