A cellulose triacetate (CTA) membrane containing trioctylmethyl ammonium chloride (TOMAC) as a carrier and o-nitrophenyloctyl ether (NPOE) as a plasticizer was developed for the in situ separation of fermented lactic acid. The separation characteristics of the CTA-TOMAC-NPOE membrane were investigated. The permeation rate of lactate through the CTA-TOMAC-NPOE membrane was comparable to that through a supported liquid membrane. TOMAC played the role of a mobile carrier in lactate transport. It was found that the CTA-NPOE membrane containing TOMAC has the advantages of allowing a high flux of lactate and having a low solubility of TOMAC in aqueous solution.