Pertraction of lactic acid (LA) through supported liquid membranes (SLM) was investigated in a spiral channel flat sheet module using a liquid membrane with ionic liquid tetradecyl(trihexyl)phosphonium bis(2,4,4-trimethylpentyl)phosphinate (IL-104) which is an effective extractant of LA. The increase in IL-104 concentration in n-dodecane from 0.32 to 0.72 kmol m(-3) did not result in an increase of the overall mass-transfer coefficient of LA, although the distribution coefficient is directly proportional to the IL concentration. According to the analysis of mass-transfer resistances, this was caused by the increased viscosity of the liquid membrane and slower diffusion of the LAH-IL-H2O complexes. Decreasing the LA concentration, the overall mass-transfer coefficient increased. This correlates well with the dependence of the distribution coefficient on the LA concentration. An increase in temperature from 25 to 35 degrees C increased the mass-transfer coefficient by about 50-70% although the distribution coefficient is independent of temperature. This was mainly due to the increased rate of decomposition of the LAH-IL-H2O complexes on the stripping interface and only marginally to the moderate decrease of SLM viscosity. The back transport of water in the pertraction of LA through SLM with phosphonium IL was identified experimentally for the first time. The new mechanism of LA transport through SLM is suggested. The transport of LA is closely related to the back transport of water in the reverse micelles which are formed on the stripping interface. Depending on the LA concentration, about 14-8 mol of water are transported back per 1 mol of LA transported forward. The reverse micelles are split on the feed/SLM interface releasing water to the feed. SLM retained its stability and initial performance during the 5.3 days long experiment, which is promising. (c) 2008 Elsevier B.V. All rights reserved.