The mixing and mass transfer characteristics of draft-tube airlift bioreactors (DTAB) for a water-in-kerosene microemulsion, as a cold model of petroleum biodesulfurization, were studied. Incomplete gas disengagement at the top-section of the DTAB and hence high gas recirculation were obtained with the microemulsion system for all the top-section configurations employed in the present study especially at the high airflow rates. The ratio (S) of the volumes of the riser and the downcomer to the top-section together with the gas disengagement abilities of the gas separator were both found to affect the mixing performance of the DTAB employed for the microemulsion system. Increase in the draft-tube height resulted in significant increase in the mixing time (t(m)) and a slight increase in the overall volumetric oxygen transfer coefficient (k(L)a). Increase in the diameter of the top-section and the height of the liquid above the draft-tube led to a decrease in k(L)a, the latter effect being less prominent. New correlations were developed that predicted the mixing time and oxygen transfer coefficients obtained in the present work with reasonable accuracy. (C) 2004 Society of Chemical Industry.