The extraction of nimbin from neem seeds using supercritical carbon dioxide is investigated in this paper. A model that accounts for intraparticle diffusion (De) and external mass transfer of nimbin (k(f)) is presented for the supercritical extraction process. Mass transfer is based on local equilibrium adsorption between solute (nimbin) and solid (neem solid). The external mass transfer coefficient was determined by fitting the theoretical extraction curve to experimental data. The following range of conditions: 0.24-1.24cm (3)/min of CO2, 10-26 MPa, 308-333 K, 1.0-2.5 g of neem kernel powder and 0.0575-0.185 cm of particle size of neem kernel powder, were considered. In addition, a new correlation for Sherwood number (Sh) was developed in terms of the dimensionless groups; Reynolds number (Re) and Schmidt number (Sc) from optimisation results. This correlation was compared to previous correlations and was found to give superior results when compared to experimental data. (c) 2004 Elsevier Ltd. All rights reserved.