Production of microbial biosurfactants requires development of bioreactors enabling sufficient oxygen supply and foaming control. Gas-liquid mass transfer in a three-phase inverse fluidized bed reactor designed for biosurfactant production by Bacillus subtilis was studied. The influence of main parameters on k(L)a was investigated in model and real fermentation conditions. Oxygen transfer increased up to 175% and up to 24% with the increase of superficial gas and liquid velocities, respectively. The oxygen transfer obtained in the TPIFB bioreactor (k(L)a up to 0.015 s(-1)) was higher than in the other bioreactors used for biosurfactant production. A significant decrease of k(L)a (up to 27%) was measured during fermentation process. It was shown that this decrease of k(L)a was a result of surface tension decrease due to the production of biosurfactants. However the oxygen transfer remained important and allowed a correct oxygen supply for the aerobic B. subtilis strains. A correlation based on dimensional analysis was proposed to estimate k(L)a in function of the influencing parameters, integrating surface tension effects. This correlation allowed estimating correctly k(L)a in the presence or not of solid particles. The correlation being based on dimensionless numbers, it could help for further process control and scale-up considerations. (C) 2013 Elsevier B.V. All rights reserved.