Synergistic effect in the extraction of rare earth (RE) metals by the acid-base coupling (ABC) extractants of calix[4]arene carboxyl derivative Bu-t[4]CH2COOH (H(4)A) and primary amine N1923 (RNH2) has been investigated. The extraction of RE was enhanced by the addition of sodium cations into the aqueous phase not only in the extraction system of Bu-t[4]CH2COOH alone but also in the mixture of Bu-t[4]CH2COOH and N1923. The separation factors (SFs) indicating the extraction selectivity of adjacent RE elements became higher in the mixture system. The synergistic extracted species and reaction mechanism were deduced for the mixture system by continuous variation and slope analysis methods. The plot of synergistic enhancement coefficient R vs. the increasing atomic number showed a "double peak effect". The equilibrium constants and thermodynamic parameters (Delta G, Delta H and Delta S) were calculated by theoretical formula. The stripping of RE can be achieved by changing the acidity of the aqueous solution. Crown Copyright (C) 2008 Published by Elsevier B.V. All rights reserved.