Unsteady mass transfer to/from a single drop in the continuous phase is formulated and numerically simulated in a moving reference coordinate system by solving the motion and mass transfer equations of an accelerating drop coupled with a level set equation for capturing the interface. Numerical simulation demonstrates the evolution of mass transfer rate and average drop concentration. Numerical simulation of the flow field and the concentration field simultaneously in each time step is compared with experimental data on single drop motion and mass transfer in two typical solvent extraction systems. The numerical predictions are found in good accord with the experimental measurements. The present numerical procedure in which the flow field is solved in a coupled way with the concentration field gives more accurate prediction than the previous decoupling algorithm by the authors. (C) 2008 Elsevier Ltd. All rights reserved.