Protonation of polybenzimidazole (PBI) ion-exchange resin with mercaptoacetic acid (MAA) results in a significant improvement of its metal sorption capacity. The MAA residue, which is bound to the polymer by relatively weak acid-base interaction, permits to a significant extent the chelation and ion-exchange reactions of the base resin while providing additional metal sorption capacity through mercaptan functionality. The PBI(MAA) resin thus sorbs metal by redox reactions as well as by chelation and ion exchange, resulting in overall greater sorption capacity as compared to PBI. For example, PBI(MAA) with a thiol content of 3.1 meq / g dry resin has silver and gold saturation sorption capacities of 6.5 and 8.0 mmol / g dry resin, respectively, while the corresponding values on the PBI free-base resin are 3.5 and 4.7 mmol / g dry resin. For copper sorption, the higher capacity of PBI(MAA) over PBI is, however, observed onlt at low solution concentrations, the saturation capacities at higher solution concentrations being nearly the same, viz., 1.1 mmol / g dry resin. Being weakly bound to the resin, metal species sorbed as mercaptides are easily removed by dilute alkali, in contrast to conventional polythiol sorbents which are often difficult to strip. The exhausted PBI(MAA) resin is regenerated by stripping with dilute alkali to remove the MAA residues along with the metal mercaptides followed by re-protonation with fresh MAA. Metal ions, suc as Ag(I) and Au(III), which are reduced to the metallic state by the polymer-bound MAA, cannot however be removed easily and necessitate treatment with alkali cyanide.