Studies on the entrainment swelling of emulsion during batch extraction of lactic acid using a liquid surfactant membrane (LSM) system are reported. The concept of effective concentration of surfactant in membrane phase has been proposed, considering the high adsorption density of the surfactant at the droplet interfaces in the LSM system. The swelling caused by emulsification during the initial dispersion process was investigated. The swelling ratio was measured by a density method. A model for estimating the "emulsification" swelling ratio, F-se, has been proposed, based on a mechanism of swelling due to the entrainment of water resulting from the interfacial turbulence and emulsification in the initial dispersion process.