A new approach is proposed for the simultaneous determination of the volumetric oxygen transfer coefficient (K(L)a) and the oxygen uptake rate (OUR) in bioreactors. The methodology is based on modifications of the classical dynamic gassing-in method and the steady-state mass balance method for obtaining the values KLa and OUR, respectively. A polarographic dissolved oxygen electrode is used to monitor the oxygen concentration during a switch of aeration gas composition. The flow rate of the gas phase is not altered so that the hydrodynamics of the bioreactor are not affected during the measurement. Data obtained with this method are compared with the classical methods to show that the proposed technique is robust and reproducible. At a 95% confidence level, the proposed method produced results that were statistically identical to data obtained with the traditional methods. The proposed technique was reproducible to within 4% of the mean value at a 95% confidence level. The proposed method was further proved by applying it to a plant-scale (17-L) batch culture of the microfungus Aspergillus terreus in a fluidized-bed reactor.