Mesomorphic porphyrin, 5,10,15,20-tetrakis(4-n-dodecylphenyl)porphyrin (C12TPPH2) and the cobalt complex C12TPPCo were used as neutral carriers of ion-selective electrodes (ISEs) to investigate the mesomorphic effect in potentiometric cation- and anion-sensing membranes. The selectivity coefficiency of a potentiometric cation-sensing membrane incorporating C12TPPH2 was drastically affected by heat-induced phase transitions of ionophoric liquid-crystalline compound. This paper is the first report that proved the heat-induced phase transition of ionophoric liquid-crystal line compound in the potentiometric ion-sensing membrane. It is noteworthy that selectivity sequence of the potentiometric cation-sensing membrane is remarkable changed by the mesomorphic effect. Furthermore, mesomorphic effect on a potentiometric anion-sensing membrane incorporating C12TPPCo was also demonstrated.