The separation of zinc, cadmium and lead ions by bis(2-ethylhexyl)phosphoric acid (DEHPA) dissolved in cyclohexane has been improved in the presence of aqueous N,N'-bis(2-pyridylmethylidene)-1,2-diiminoethane (BPIE), as a selective masking agent. The interaction between metal ions and BPIE results a shift in the extraction curve of the studied metal ions versus pH towards higher pH region. The displacement of the extraction curves varies as Zn2+ < Pb2+ < Cd2+. The analysis of extraction data makes possible the evaluation of stability constant of the [M(BPIE)](2+) complexes (M = Zn, Cd or Pb) in the aqueous phase. The influence of temperature on the extraction of studied metal ions from aqueous phase in the absence and the presence of BPIE was tested in the range 288-303 K. This investigation allows evaluating the thermodynamic parameters associated with the extraction process, and those of the complexation of cations by BPIE in the aqueous phase. (C) 2008 Elsevier B.V. All rights reserved.