Reactive extraction of penicillin G from the continuous aqueous phase to the dispersed organic phase in an agitated vessel was simulated using two mass transfer models with negligible continuous phase mass transfer resistance. The calculated results from one of these two models (Model 2), in which mass transfer of penicillin G was controlled by the diffusion in organic drops and the interfacial reaction, fit the experimental data for degree of extraction better. Thanks to the mass transfer model, the chan-e in the dimensionless penicillin G concentration in the continuous phase with time was investigated for various values of Danckwert numbers at a fixed mass transfer coefficient or Biot numbers at a fixed interfacial reaction rate constant, and then it was confirmed that the mass transfer model should not include the mass transfer resistance in the continuous aqueous film.