Tetraalkylphosphonium ionic liquid (IL) with a bis 2,4,4-trimethylpentylphosphinic anion (Cyphos IL-104) is an effective extractant of lactic acid (LA) achieving at low LA concentrations the distribution coefficients for aqueous systems above 40. L/L equilibrium data for pure Cyphos IL-104 and its solutions in n-dodecane have been measured. With increasing acid concentration the value of the distribution coefficient of LA decreases. Cyphos IL-104 extracts only undissociated molecules of lactic acid (LAH) via H-bonding. Increase in the concentration of IL-104 in n-dodecane results in increasing distribution coefficient of LA and the water solubility. The high water content in the solvents with Cyphos IL-104 is connected with the formation of reverse micelles. An interesting phenomenon of the liberation of water from the solvent in extraction of LA has been observed. It is suggested that splitting of the reverse micelles due to the formation of LAH/IL complexes occurs. With increasing temperature the values of the distribution coefficients of LA practically does not change or only slightly increases. In contrary to this, the increase in temperature decreased the water solubility in solvents containing IL-104. This is interpreted by the lower stability of reverse micelles at higher temperatures. Based on experimental equilibrium data the formation of stoichiometrically defined complexes with the structure (LAH)(p)(IL)(H2O)(2) and (p, 1, 2) stoichiometry, where p is in the interval from I to 3, is suggested. The proposed model fits the equilibrium data well and indicates the domination of the (2, 1, 2) complex at medium aqueous acid concentrations in the interval from 0.2 to 2 kmol m(-3). There are two mechanisms of water extraction into the solvents with Cyphos IL-104: the formation of reverse micelles, and the formation of hydrated complexes of LAH with IL. A sharp decrease in the viscosity of solvents with IL-104 with increasing concentration of water or LA was observed. (C) 2006 Elsevier B.V. All rights reserved.