The separation of thorium, lanthanum and europium ions has been improved by acidic extractant bis(2-ethylhexyl)phosphoric acid (DEHPA) in cyclohexane using 2-nitrobenzo-18-crown-6 (NB18C6) as a selective masking agent. The presence of NB 18C6 shifts the extraction curve of lanthanide ions versus pH towards higher pH region. NB 18C6 was efficient masking agent towards lanthanide ions and a better separation was obtained as compared to 18C6. The stability constants of the lanthanide-crown ether complexes in aqueous phase and the thermodynamic parameters associated with the extraction process were evaluated. (C) 2004 Elsevier B.V. All rights reserved.