In an internal loop airlift reactor of 55L working volume, the gas-liquid volumetric oxygen mass transfer coefficient k(L)a, gas holdup epsilon(G) and liquid circulation time t(c) were measured with the solution of carboxymethyl cellulose (CMC) to simulate the performance of a reactor with highly viscous broth. Electric conductivity and oxygen probes were used to measure the local gas holdup, liquid circulation time and oxygen mass transfer coefficient in the individual sections of the reactor (riser, downcomer and the gas-liquid separating section at the top of the reactor) and the total reactor, respectively. The values of k(L)a for the riser, downcomer and separation sections of the reactor were also estimated and compared with that for the total reactor. The results show that, both k(L)a and epsilon(G) increase but t(c) decreases with increasing gas velocity. Correlations and comparisons with works reported in the literature are also presented. Data show that the methods developed for k(L)a measurements in the individual section and total reactor are applicable to the studies of internal loop airlift reactors.