Polyethylenimine (PEI) was modified with ligands containing sulfur donors to give soluble polymers for binding toxic metal ions. Reaction of purified PEI with mercaptosuccinic anhydride, ethylene sulfide, or methylthiocyanate gave PEI-MSA (25% functionalization), PEI-ET (100% functionalization), and PEI-TU (25% functionalization), respectively. Purification of the polymers was accomplished by diafiltration. The capacities for toxic metal ions (Hg, Cd, and Pb) and transition metal ions (Cu and Ni) were measured for each of the polymers. PEI-ET and PEI-TU showed high affinity for the softer metal ions, Hg and Cd, with loading capacities substantially higher than those for the base polymer PEI. Both polymers had high capacities for Cu. Release of the metal ions from the polymers was accomplished by lowering pH; however, small amounts of metal remained bound to the polymers at pH 1. Competition studies showed that PEI-TU and PEI-ET bound Hg and Cu more strongly than Cd and Pb.