Models were developed for prediction and interpretation of the observed steady-state axial dissolved oxygen concentration profiles in tall airlift bioreactors. The observed concentration profiles were non-linear because of a combination of hydrodynamic and mass transport factors. The profiles were influenced mainly by the liquid-phase axial dispersion coefficient, the volumetric overall gas-liquid mass transfer coefficient, the gas velocity, the induced liquid circulation velocity. The model-predicted concentration profiles agreed within +/-2% with the measured data in a tall (working aspect ratio similar to 15) airlift vessel operated under aeration regimens that are typically used during wastewater treatment. Axial inhomogeneities in dissolved oxygen increased with increasing aeration rate. This phenomenon may influence activated sludge processes in airlift and deep-shaft reactors. The maximum attainable concentration of dissolved oxygen at the bottom of a typically aerated airlift reactor, greater than or equal to 3.5 m deep, always remained at < 80% of air saturation value even when oxygen was not being consumed. Also, at steady state and without a net transfer of oxygen, the gas-phase mole fraction of oxygen varied by > 10% axially up the reactor.