Dispersed water-immiscible solvents are known to enhance oxygen transfer rates in oxygen-limited aerobic fermentations. Here, this technique is applied to improve the mass transfer rate of poorly water-soluble gaseous pollutants during the biological treatment of waste gases. In a stirred-tank reactor, the enhancement of mass transfer rates was studied as a function of the pollutant solubility in water. The solvent used was FC40 (up to 10% v/y) and the model gaseous pollutants were toluene and oxygen (moderately and poorly water-soluble, respectively). The overall volumetric mass transfer coefficient from the gas to the bulk liquid (k(i)a(gl)) was measured under nonsteady-state conditions in the absence of micro-organisms. It was found to be essentially constant for the solvent volume fractions tested and for both toluene and oxygen. Using the values of k(i)a(gl) and the partition coefficient gas/liquid (m(gl)), the enhancement of the mass transfer rate by solvent addition could be predicted theoretically. A good agreement between the theoretical evaluation and the experimental results from experiments in the presence of biological consumption was observed. An enhancement of the mass transfer rate by a factor of 1.1 was found for toluene using a dispersion containing 10% (v/v) FC40 while the oxygen transfer rate increased by a factor of two at the same solvent volume fraction. It was further demonstrated theoretically for other gaseous compounds that the addition of solvent has a more pronounced effect on the enhancement of the transfer rate in the case of poorly water-soluble compounds compared to moderately water-soluble ones.