In a boundary-fitted orthogonal coordinate system, the general governing equations for steady and transient axisymmetrical cases of mass transfer in the external region around single buoyancy-driven drops in steady motion under low or intermediate Reynolds number were expanded and numerically solved by the control volume formulation. The simulation results for typical cases were presented and compared well with the data and empirical correlations in the literature. It suggests that the developed mathematical formulation of the external mass transfer for a deformable buoyancy-driven drop is reasonable, and may give reliable simulating results. The analysis of the mass transfer coefficient against the detail of flow near the drop interface shed light to the respective contribution of molecular diffusion, convection and recirculating wake, as to provide information on the characteristic time scale of decaying Sherwood number, and hence, to be Valuable for analyzing and optimizing the solvent extraction operation. (C) 2001 Elsevier Science Ltd. All rights reserved.