The metal-complexing properties of five different polyaziridines-poly[I-(2-aminoethyl)aziridine], poly( 1-acetylaziridine), poly[ 1-(2-hydroxyethyl)aziridine], poly[l-(2-hy-droxyethy1)aziridine-co-1-acetylaziridine], and poly[ 1-(2-aminoethyl)aziridine] oxine-were investigated in an aqueous solution with regard to the side-chain effects using membrane filtration. The results are discussed and compared with the stability constants of metal complexation of the corresponding functional moieties. Poly[ 1-(2-aminoethyl)aziridine], a poly(aziridine) with neutral nitrogen donors in the side chains, showed similar complexing properties to those of ammonia. The only exceptions are Fe(II1) and Cr(III), which cannot form complexes with ammonia but can be retained in the membrane filtration process due to the formation of Fe(II1)-colloidal species and hydroxides. The complexing properties of polyaziridines containing hydroxyl groups in the side chains (i.e., poly[l-(2-hydroxy-ethy1)aziridinel and poly[ 1 -(2-hydroxyethyl)aziridine-co-l-acetylaziridine] can be explained and described by the formation constants with the corresponding hydroxides. The introduction of a carbonyl moiety in the polyaziridine side chain, e.g., polyacetylaziridine, reduces the complexing ability for metal ions because the carbonyl group cannot form complexes with the metal ions investigated. Poly[ 1-(2-aminoethyl)aziridine]oxine showed the strongest complexing ability in the series of metal ions due to the strong complexing ligand 8-hydroxy quinoline attached to the polymer backbone.