The influence of power consumption on the volumetric mass transfer coefficient was studied in a 5 liter stirred tank bioreactor during cultivation of a recombinant Chinese Hamster Ovary (CHO) cell line, which requires low aeration and mixing intensity. Under these conditions and at high cell concentrations, oxygen mass transfer coefficient and consequently volumetric oxygen mass transfer rate was not sufficient for the oxygen requirements. This problem was successfully solved by introducing pure oxygen into the bioreactor and the oxygen mass balance was studied. For that purpose, respiration rate as a function of cell concentration, as well as volumetric oxygen mass transfer rate at different operating conditions were measured. Graphical presentation of the results show the process control possibilities and the role of mixing intensity necessary to ensure that there is sufficient oxygen supply for a given cell concentration and respiration rate.