A novel host compound calixarene has been applied as a mobile carrier for the separation and concentration of rare earth metals by liquid surfactant membranes (LSMs). Using the novel carrier, extraction of rare earth metals (Ho, Er and Y) by LSMs was systematically studied in a stirred cell. Extraction behavior of rare earth metals by the carboxyl derivative of calix[4]arene was found to be extremely sensitive to coexisting sodium ions in the feed solution. The extractability of rare earth metals was remarkably enhanced by the addition of sodium ions. The permeation rate of rare earth metals through the LSMs in the presence of sodium ions was 100-fold higher than that without sodium ions. This enhancement of the permeation rate is attributed to the preferential conformation change in the host compound of calixarene initiated by sodium ions. The permeation mechanism of rare earth metals by LSMs is elucidated by an interfacial reaction model. (C) 1998 Elsevier Science B.V.