The mass transfer into droplets in metal extraction is investigated in the system ZnSO4-H2SO4/di(2ethylhexyl)phosphoric acid in isododecane. Results of equilibrium measurements and stirred cell experiments are described in a mathematical model based on gradients of chemical potential. Concentration profiles measured with Laser Induced Fluorescence technology were calculated with the Maxwell-Stefan equations. Diffusion coefficients were estimated using the Wilke and Chang equation. Additional eddy diffusion coefficients measured in single droplet experiments in a venturi tube are described with different models known from literature. Whereas equilibria and mass transfer into droplets in stagnant conditions can be calculated with good agreement with the experimental results, none of the known models describes eddy diffusion in reactive extraction occurring in free moving droplets sufficiently well over a broad concentration range. (C) 2002 Elsevier Science B.V. All rights reserved.