Higbie’s penetration theory is combined with a model of gas hold-up in order to set up an expression for the prediction of the volumetric oxygen mass transfer coefficient, K(L)a, in airlift bioreactors. The results indicate that the K(L)a is a strong function of the superficial gas velocity, gas hold-up, liquid circulation velocity, bubble diameter and bubble rise velocity. A 2501 pilot-scale concentric cylinder airlift bioreactor having a shell diameter of 0.317 m and a draught tube diameter of 0.211 m is used to obtain local measurements of the K(L)a as a function of the superficial gas velocity in the riser during the fermentation of Saccharomyces cerevisiae. The results show that in the downcomer, K(L)a, increases with increasing height of the liquid from the sparger while in the riser the K(L)a is relatively uniform and has a value similar to that observed in the lower section of the downcomer. The K(L)a measurements are described and discussed using the proposed model.