The extraction of lactate by an ion-pairing mechanism using a quaternary ammonium salt (Aliquat 336) was studied previously and a mathematical model for equilibrium was developed. The evaluation of the equilibrium constant (K-e) of the reaction between carrier and lactate allowed a good prediction of equilibrium for independent extraction and stripping, within a large range of experimental conditions. However, when extraction and stripping operations are carried out simultaneously using liquid membranes and different concentrations of the feed and stripping solutes are used, the resulting osmotic pressure difference between the two aqueous compartments (feed and stripping) has to be taken into account for equilibrium prediction, if the membrane is not totally impermeant to water. Model prediction of equilibrium and identification of the mechanisms involved on the transport of lactate and of the ion counter transported are presented. The influence of operating conditions on each mechanism and their relative contribution to the overall transport are evaluated. Prediction of equilibrium is accurate in the absence of an initial osmotic pressure difference between the two aqueous compartments but exhibit a large deviation for increasing initial chloride concentrations. To counterbalance the initial osmotic pressure difference an increasing hydrostatic pressure difference builds up leading to salt transport across the liquid membrane.