To investigate the performance of external-loop airlift reactors, the gas holdup and the volumatric mass transfer coefficient were determined experimentally in an airlift reactor with Ca-alginate gel beads. The results show that the overall gas holdup and liquid-phase mass transfer coefficient increased with increasing gas velocity but slightly decreased with increasing particle concentration. The particle size had no clear effect on the variation of gas holdup or oxygen transfer. A mass transfer model was proposed based on Higbie's penetration theory and Kolmogoroff's isotropic turbulence theory, and it compared fairly with experimental data.