Calixarene amides incorporated in poly(vinyl chloride) membranes provide good Na+ selectivity for CHEMFETs. A remarkable difference in the Na+/Ca2+ selectivity was observed depending on the presence or absence of tetraphenylborate ions in the membrane matrix. In the latter case the potentiometric selectivity coefficient (log KNa,CaPot) increased from −2.1 to −3.6. Calixarene amides proved to be excellent ionophores to obtain Na+ selectivity in the presence of K+, i.e. log KNa,KPot = −2.7.