Chlorosulfonated polystyrene-DVB (10%) copolymer (in beads form, 720-840 mu m) has been modified with triethylenetetramine (TETA) to ave the corresponding polymeric sulfonamide. By treating with the potassium salt of chloroacetic acid in water, amino groups of the polymer have been carboxymethylated almost quantitatively. The resulting: material with iminoacetic acid pendant groups has very efficient chelating ability for Ca(II) and Mg(II) ions in ppm levels. The chelating polymer can be regenerated ten times by acid leaching without losing its original reactivity. It is suitable for removal of calcium and magnesium ions to supply soft water.