Based on the extraction mechanism by carriers in the membrane phase, the hydrophilic property of the carrier-solute complex formed at the interface of emulsion has been studied. By gas chromatograph the solubility of water in the membrane phase was measured. Solubilization of water in the membrane phase is caused not only by surfactant but also by carrier-solute complex. Furthermore, water diffusion through the membrane goes on due to both the chemical potential of electrolytes and the chemical potential between reactants and reaction products. A mathematical model for estimating the permeation swelling rate during lactic acid extraction is proposed. With this model, the calculation results are in good agreement with the experimental data.