A major goal of our research program is to develop water soluble chelating polymers for the selective separation of trivalent cations such as iron(III) and tetravalent actinide ions such as Th(IV) or Pu(IV) from aqueous, radioactive waste streams. As part of this effort, we have synthesized a new water soluble polyethyleneimine polymer that incorporates N-methylhydroxamic acid chelating groups. Both the synthesis of this chelating polymer, PEI-NMH, and its ability to achieve selective separation of target metal ions using ultrafiltration will be discussed. It is interesting to note that the PEI-NMH polymer selectively binds iron(III) over thorium(IV) at pH I in contrast to its primary hydroxamate analog PEI-H. The binding properties of these polymers have been evaluated using both potentiometric and spectrophotometric methods. A comparison of the binding properties of PEI-NMH and PEI-H reveals the importance of the nature of the ligand in the complexation process. (C) 2002 Elsevier Science B.V. All rights reserved.