A promising technique for quantification of the mass transfer coefficient k(l)a for oxygen in bioreactors is described. The method is based on injection of the volatile, inert Kr-85 isotope into the medium followed by measurement of the radioactivity in the gas leaving the head space. The measured response is interpreted using a simple model for the gas flow through the bioreactor. The method is compared with two other methods : (1) a dynamic method based on N2 and (2) the classical sulphite method. The isotope method compares well with the dynamic method and, from the comparison with the sulphite method, it is concluded that the sulphite method gives an overestimation of k(l)a which can not be explained solely by reduced coalescence due to the electrolyte. The extra effect is probably due to chemical reaction in the liquid film. The isotope method has been used to study the influence of the medium composition on the oxygen mass transfer. A major advantage of the Kr-85 method is that it can be applied during real process conditions as illustrated in an experiment with growth of Aspergillus oryzae.