Interfacial reaction kinetics for reactive extraction of penicillin G from aqueous solutions by Amberlite LA-2 in a mixture of kerosene and n-butyl acetate were studied using a microporous membrane-based stirred cell. This operation gave intrinsic rate equations for formation and dissociation of the Amberlite LA-2 complex of penicillin G occurring at or near the organic-aqueous interface. The effect of temperature on the reaction rates was also examined and the activation energies were determined. On the basis of the reaction kinetics and transport parameters within two stagnant layers, the fractional resistances of aqueous layer diffusion, interfacial chemical reaction, and organic layer diffusion were quantitatively determined in this liquid-liquid extraction process (forward and backward), and thus the transport mechanism was clarified.