The fabrication and characterization of a combined pO2, pCO2 and pH chemical sensor, designed for blood-gas monitoring, are discussed. Classical electrochemical principles are used in a miniaturized planar-type structure. Both amperometric (pO2) and potentiometric devices (pCO2 pH) are integrated on a 10 mm × 10 mm chip. The transducer part of the chip is realized using standard silicon technology. Polyacrylamide and polysiloxane layers, which are used as hydrogel and gas-permeable membrane, respectively, are deposited and patterned by photopolymerization. Thus the whole sensor is fabricated on wafer level using IC-compatible processes. The characterization has been performed in aqueous solutions and in blood used for transfusion. For this purpose, the chip is mounted into a flow-through cell. The sensors have a typical sensitivity of 5 nA/mmHg (pO2), 50 mV/decade (pCO2) and 55 mV/pH (pH).