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) alpha 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 uL 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.