The present invention provides an ion selective electrode for anions such as Cl.sup.- and cations such as Ca.sup.2+ which has a sensitive membrane comprising a polymer material and containing an ion sensitive material and a plasticizer. In thins ion selective electrode, the ion sensitive material is an ion exchanger such as a quaternary ammonium salt, a quaternary phosphonium salt or an organic phosphate and the plasticizer is a compound having the formula (1): ##STR1## wherein X represents an alkyl group, a phenyl group or a phenyl group to which a functional group bonds. As the plasticizer there may also be used a mixture of the compound of the formula (1) with an alcohol compound such as an alcohol having a dielectric constant of 10 or less and containing 10 or more carbon atoms or an ester compound.