A membrane sensitive to anions is obtained by forming into a membrane either a composition comprising an onium salt compound such as trioctylmethylammonium chloride or tetraoctylammonium bromide, and an aromatic boric diester compound such as (p-alkyloxy)phenyl borate, or a composition which comprises these two ingredients and a membrane-forming polymer such as polyvinyl chloride or polystyrene, and may further contain a fat-soluble anion salt such as tetraphenyl borate. This membrane can yield an ion-selective electrode which permits hydrogencarbonate ions contained in body fluids to be rapidly determined with high sensitivity and high selectivity and which has a long life.