The direct reactive extraction of propionic acid from Propionibacterium acidipropionici broths with solutions of tri-n-octylamine in dichloromethane, n-butyl acetate or n-heptane underlined the strong negative influence of the cells, due to the blockage of the interface by their adsorption. The magnitude of this effect depends on the affinity of the cells for the organic phase, which is more important for n-heptane, but only at biomass concentrations below 18gL(-1) d.w. (dry weight). Moreover, the interfacial mass transfer of the acid is also controlled by the solvent polarity, and is accelerated from n-heptane to dichloromethane and by the addition to the organic phase of 1-octanol as a phase modifier. The influences of the biomass concentration, the rotation speed and the solvent dielectric constant were included in a mathematical model describing the solute mass flow from the aqueous to the organic phase.