Abstract: A new substrate for the construction of solid internal contact ion-selective electrode comprising a conductive and porous carbon matrix is described. The well-defined (i.e., fixed) interfacial potential between the organic electroactive phase and the internal reference element, coupled with the electrode's very slow change from water activity at the internal reference element, supports the well-behaved and reproducible electrode operation. Solid contact sensors for two different ions were constructed, and their potentiometric behavior was compared with their symmetric counterparts. The potentiometric response was monitored over a period of one month with excellent results for drift, base line stability, response time, selectivity, and detection limit.