A simplified model that considered the oil constituents as one constituent, anethole, the major component, was used to describe the mass transfer of steam extraction of aniseed essential oil. The model can be used to optimise and control the process. Depending on the oil content, two mass transfer regimes were identified (i) the first one corresponds to an unsaturated surface extraction and (ii) the second corresponds to the slower transfer of oil from the deeper parts of the material to the surface, which may be due to concentration gradients and chemical bonding. The model was validated by experimental data obtained from a pilot-plant system. Solid-steam mass transfer coefficients were determined and a critical oil content was found to limit the two mass transfer regimes. The value for this critical oil concentration (x(B)) was found to be 0.011 (g oil g(-1) solid). In addition, an optimal operating pressure of 200 kPa was found to give maximum extraction yield. (c) 2005 Society of Chemical Industry.