The liquid-liquid extraction of three carboxylic acids (valeric acid, malic acid and lactic acid) from aqueous solutions using a tubular UF ceramic membrane as phase separator (extractive ultrafiltration) has been investigated. The extraction mechanisms for the more effective extracting systems among those tested for each organic acid are explained. Extraction results show the contribution of the physical extraction by the diluent and the reactive extraction by the extractant, with the formation of different acid-extractant complexes, whose stoichiometries can be related to the physico-chemical characteristics of each acid. Equilibrium constants were obtained for all the partition and complex formation reactions involved in the extraction mechanisms. The extractive ultrafiltration - a separation technique, which combines an extraction step with a membrane filtration step - of the aforementioned acids was carried out using three operating modes (total recycle, concentration and continuous modes). A different behavior depending on the nature of each acid (and the complexes they form during the extraction process) was found, showing that solute-membrane interactions play an important role. The effect of transmembrane pressure on permeate flux during the experiments carried out at total recycle show linear trends for all conditions tested. Concentration experiments allowed obtaining the maximum organic to aqueous phase ratio achievable for the three case studies (with the best extraction systems). From an initial 0.1 ratio, values of 0.88, 0.62 and 0.75 could be obtained for valeric, malic and lactic acids, respectively. Finally, a continuous operation showed the evolution of flux and acid concentration in both the aqueous permeate and the rejected emulsion as the organic phase becomes saturated. (c) 2005 Elsevier B.V. All rights reserved.