In this study, a general mass transfer model for extraction with liquid surfactant membranes (LSMs) is proposed by using the operator theory. This model, incorporating different parameters, may account for the mass transfer mechanisms for both type-I- and type-II-facilitated transport, and the results are consistent with zero-order perturbation solutions. The model with parameters for type I is verified with the experimental data of O’Brien and Senke (1989, Sep. Sci. Technol. 24, 617-628), while the model with parameters for type II, with the author’s experimental data, showing that both selectivity and separation efficiency for type II are superior to those of type I.