A mathematical model for analyzing the experimental data of extraction of weak acid from aqueous solution by liquid surfactant membrane (LSM) using a strong base as internal reagent present in excess in a batch system has been presented. The leakage of internal phase due to membrane breakage is also discussed in the mathematical treatment. The model while considering a reaction front to exist within the emulsion globule assumes reaction equilibrium between the solute and internal reagent in the external continuous phase. The proposed model predicts successfully the experimental results of extraction of a weak acid by a strong base in a batch separation system as presented in the literature. The model is also capable of predicting excellently the experimental pH versus time data in case of the above system.