The liquid phase mass transfer coefficient k_L for an external loop airlift has been investigated to elucidate a mechanism and degree for the gas–liquid concurrent, circulating liquid velocity u_L to affect it. The approach is based on the Higbie model, i.e. k_L∝u_S^0.5 at a fixed average bubble diameter d_B, where the average bubble slip velocity u_S is derived from the drift flux model. The u_S and k_L values have been deduced to decrease in the order of the gas–liquid concurrent flow, batch liquid and countercurrent flow. The result is elucidated by variation of the bubble wake development with the turbulent liquid flow direction and rate, in addition to variations of liquid inertia and drag on individual bubbles. The observed k_L, surpassing k_LS for the single bubble and k_L,BF for the bubble flow (BF), increases from the enhancement effect of concurrent u_L on k_L through its preventing bubbles from clustering, coalescence and breakup. The k_L values could reasonably be reproduced by the Higbie model in which the terminal rise velocity u_BS is replaced by u_S,BF=(gd_B/2)^0.5 for the BF. A factor F=k_L/k_L,BF derived from the above u_S,BF-based equation for k_L well reproduces the observed F.