A mechanistic model capable to accurately describe the mass transport of hexane in a two-liquid phase partitioning bioreactor (TLPB) constructed with silicone oil was developed. This work constitutes the first step in the development of simple and reliable models for the mathematical description of the off-gas treatment of volatile organic contaminants in TLPBs. The model (based oil general mass balances and transport equations over off-gas rising bubbles) predicted a negative linear relationship between the fraction of organic phase and the logarithm of the pollutant fraction that remains in the gas phase The average relative errors of the model predictions were lower than 7%. Under the tested range of operational conditions (organic phase fractions and stirring rates ranging from 5% to 30% and from 100 to 300 rprn, respectively) the Maximum hexane fraction transferred from the gas to the liquid (aqueous + organic) phase increased at increasing stirring rates and silicone oil fractions up to 200 rprn and 20%, respectively. In addition, the proposed modelling approach quantified the proximity of the dispersion to thermodynamic equilibrium conditions. predicting thus the degree of mixing in the systems (C) 2009 Elsevier Ltd All rights reserved.