Abstract: A method for demineralizing water or an aqueous solution, which comprises contacting the water or the aqueous solution to be treated to a strongly basic anion exchanger made of a crosslinked polymer having a constituting unit of the following formula (I): ##STR1## wherein A is a C.sub.1-2 linear alkylene group, B is a C.sub.4-8 linear alkylene group, each of R1, R2 and R3 which may be the same or different, is a C.sub.1-4 alkyl group or a C.sub.2-4 alkanol group, X is a counter ion coordinated on the ammonium group, and the benzene ring D may have an alkyl group or a halogen atom as a substituent.

Abstract: An anion exchange method conducted at a high temperature, which comprises using a crosslinked anion exchanger which comprises structural units having a quaternary ammonium group of the following formula (a) and structural units derived from an unsaturated hydrocarbon group-containing crosslinkable monomer, and of which at least 90% of anion exchange groups are present in the form of a group represented by ##STR1## as defined in the following formula (a): ##STR2## wherein R is a --(CH.sub.2).sub.n --group (n=an integer of from 3 to 18) which may include a cyclic hydrocarbon, each of R.sub.1, R.sub.2 and R.sub.3 which are independent from one another, is a C.sub.1 -C.sub.8 hydrocarbon group or a C.sub.1 -C.sub.8 alkanol group, the benzene ring of the formula (a) may be substituted by an alkyl group or a halogen atom, or may be condensed with other aromatic ring, and X is an anion.