Poly(N-2-sulfoethylethylenimine) (SEPEI) with a degree of modification equal to 0.30, 0.58, and 0.74, cross-linked with diethylene glycol diglycidyl ether, demonstrates efficient sorption properties with respect to PtIV, PdII, and AuIII ions. The selectivity coefficient KPd/Au in a HCl solution with pH 0.8 decreases from 90 to 61, and the selectivity coefficient KPd/Pt in a HCl solution with pH 3.9 increases from 0.94 to 480 with an increasing degree of modification. A thiourea hydrochloric acid solution effectively removes metal ions; the desorption of PtIV, PdII, and AuIII from the SEPEI surface reaches 100, 96.9, and 83.8%, respectively.