The aim of this work is to evaluate the gadolinium(III) rejection by nanofiltration assisted by complexation with diethylenetriaminepentaacetic acid (DTPA) as chelating agent. The experimental Gd(III) rejections as a function of the pH were compared to species distribution for the Gd(III)-DTPA system calculated using the protonation and thermodynamic stability constants. The Gd(III) rejection varies from 5 to 95% as a function of the pH and follows the Gd(III)iDTPA speciation. However for pH below 1.5 and for high Gd(III) rejection, some differences between the Gd(III)/DTPA speciation and the observed Gd(III) rejection could be noticed. For this reason, the surface charge of organic NF membrane Desal G10 was determined from tangential streaming potential measurements for different pH values (2.0-6.0) in the presence of 0.5 mM of Gd(III) nitrate and 0.5 mM of DTPA. The isoelectric point (pI) of the Desal G10 membrane was found to be close to 3.4. Thus the rejection of Gd(III) at low pH could be explained by surface force interactions. The influence of the operating parameters on Gd(III) rejection above pI was then studied. For an operational pressure of 3 bar, a complex concentration of 0.4 mM, a tangential velocity of 145 mm s(-1) and a temperature of 15 degrees C, the differences between the Gd(III)/DTPA speciation and the observed Gd(III) rejection are low and thus Gd(III) rejection with Desal G10 membranes can fit the speciation. (c) 2005 Elsevier B.V. All rights reserved.