The fabrication process for a potentiometric silicon sensor will be described. The sensor was fabricated in bulk silicon micromachining using a double side wafer process. A small channel was etched anisotropically in 100 silicon. The back side groove was metallised. To form an ion-selective electrode an ion-selective membrane was deposited in the channel. Additionally, a novel type of Ag/AgCI/Ag reference electrode was integrated on the front side of the chip. The small size of the sensor allows the application in catheter systems.