The invention relates to a method for removing a positively charged substance from an aqueous liquid (I) by contacting the liquid with a cation-exchanger (1), possibly followed by a subsequent desorption of said substance. The cation-exchanger is selected to be capable of (a) binding to said substance by cation-exchange in an aqueous liquid reference (II) at an ionic strength corresponding to 0.3 M NaCl and (b) permitting a break through capacity for said substance .sup.3200%, such as .sup.3300% or .sup.3500%, of the break-through capacity of said substance for a reference cation-exchanger (2) containing sulphopropyl groups --CH2CH2CH2SO2O--. The cation exchange ligands have an at least bimodal function by comprising a cation exchanging group and a separate hydrogen-bonding atom. The invention also relates to a method for testing the appropriateness of a cation-exchanger for removing a substance from a liquid and novel cation exchangers.