Anion exchange method

A demineralizign process conducted at a high temperature condition of above 60.degree. C., which comprises treating water with an ion exchanger containing a crosslinked anion exchanger wherein said 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 ##STR16## as defined in the following formula (a):

• ##STR17## 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.