The present invention relates to method of manufacturing a sulphonate-functionalized (S-functionalized) cation exchanger, which method comprises reacting olefinic groups with bisulphite in the presence of at least one amine oxide, such as N-methyl morpholine oxide (NMO). The olefinic groups of the carrier may be allyl groups; which may be provided on extenders such as polyhydroxyfunctional polymers. In one embodiment, the carrier is made of agarose with improved flow pressure properties.