A planar miniaturised CO2 sensor based on thick films of Nasicon (Na+ conductor, Na3Zr2Si2PO12) electrolyte has been developed. The thick film was fabricated by screen printing Nasicon paste on an alumina substrate and then firing at 1543 K. scanning electron microscopy (SEM) analysis shows that the films achieved good densification, although reactions occur between Nasicon and alumina substrate, so conductivity of the thick film is lower than that of bulk Nasicon. The sensors interfaced with a binary carbonate auxiliary phase (Na2M-BaCO3) applied as a slurry at 373 K showed good CO2 sensing properties at 673-873 K. The electromotive force (emf) values obtained were linearly dependent upon the logarithm of CO2 concentration over a wide range (6 ppm-100%). The slopes of emf versus logP(CO2) agreed well with the Nemstian slopes based on two electrons electrode reactions.