The volumetric mass transfer coefficient (k(L)a) of oxygen was measured in internal loop airlift reactors of different scale using the dynamic pressure-step (DPM) method. The k(L)a values obtained by the DPM appear as the most reliable as they were found to be independent of the oxygen concentration in the inlet gas no matter the reactor scale. The effect of reactor scale on k(L)a values was studied in three airlift reactors of different working volume (12, 40 and 195 dm(3)) but with similar geometric configuration. The ratio between the riser and the downcomer cross-sectional areas and the slightness of the column were taken as similarity criteria. As the liquid phase deionized water was used at a temperature of 27.5 degrees C. Both the gas hold-up (epsilon(G)) and the mass transfer coefficient (k(L)a) increased with increasing superficial gas velocity. The experimental dependencies of the k(L)a and epsilon(G) values versus the superficial gas velocity were very similar, what indicated a strong dependency of the k(L)a values on gas hold-up. The variation of the mass transfer coefficient with the gas hold-up was described using an appropriate correlation taken from the literature. The parameters of this correlation were almost the same for all working volumes of the reactor. Thus, it was found that even in larger reactors only the gas hold-up is the main key parameter influencing the k(L)a values. (c) 2006 Elsevier B.V. All rights reserved.