The binding of Cu2+, Cd2+, Co2+, Pb2+, Zn2+, Ni2+, Cr3+to poly(sodium 2-(N-acrylamido)-2-methyl-propanesulfonate) (PAMPS) in the presence of poly(acrylic acid) (PAA) is studied by diafiltration. It is found an unusual behavior for the polyelectrolyte PAMPS, since it binds irreversibly a fraction of metal ions, while the rest of the ions are eluted form the diafiltration cell without interacting with the polyanion. At pH 3, and in the presence of PAA, the fraction of metal ions bound to the polymer decreases, as the sulfonate molar fraction with respect to the carboxylate groups decreases. A copolymer composed by sodium 2-(N-acrylamido)-2-methyl-propanesulfonate and acrylic acid showed the same binding ability than a mixture of PAMPS and PAA under the same relative sulfonate / carboxylate composition