The effects of temperature, agitation speed, aeration rate and the configuration of the cage screen on the volumetric oxygen transfer coefficient (K(L)a) were investigated in a 20-l modified cell-lift bioreactor with a double-screen cage (CellCul-20A). The results showed that the K(L)a value increased significantly with an increase of aeration rate. Based on the experimental data, where possible the K(L)a value was correlated with each operating variable (e.g., the aeration rate). For a better characterization of the oxygen transfer process in the bioreactor, first, the liquid exchange rate across the screen of the cage (Q(s)), which was evaluated by an inhouse-developed electrolyte pulse technique, was successfully correlated with the agitation speed and aeration rate as follows : Q(s) = 3.42 x 10(-5) N-1.49 + 1.46 x 10(-4) U-G(0.68) (for a double screen) and Q(s) = 1.93 x 10(-5) N-1.81 + 1.13 x 10(-4) U-G(0.61) (for a single screen). The following mathematical model for the oxygen transfer in deep aeration was then developed : 1/[(K(L)a)d . V-c] = 1/Q(s) + 1/{V-b .[(K(L)a)b .(K(L)a)d]}. Using this model, the value of the volumetric oxygen transfer coefficient inside the cage was obtained, and the mass transfer resistances for the bubbling inside the cage and the transfer of liquid across the screen of the cage were compared. The results provide an insight towards a better understanding of the mass transfer mechanisms in the bioreactor.