A new approach for volumetric oxygen mass transfer determination in an upflow cocurrent bioreactor packed with expanded clay balls is proposed in this paper. The chemical method of CuCl oxidation was adapted for k(L)a determination in the Biological Filtration Oxygenated Reactor (BIOFOR(R)). The variations of water and air superficial velocities showed that the water superficial velocity significantly influences the oxygen mass transfer. However, variation of air superficial velocity has little effect on the mass transfer coefficient. A mathematical procedure is developed for k(L)a calculation using experimental results. To predict the volumetric oxygen mass transfer coefficient, a modified correlation, published by Reiss (1967, I&EC. Proc. Des. Dev. 6, 486-499), is proposed. Using water superficial velocities and pressure drop, this correlation enables the determination of the k(L)a coefficient at any time, during the biological wastewater treatment cycle.