A planar type CO2 gas sensor employing (8 mol% Y2O3) ZrO2 (YSZ) thin film on Na3Zr2Si2PO12 (Nasicon) substrate with Na2CO3 as an auxiliary electrode has been fabricated and tested in laboratory environment between 700-900 K. The YSZ thin film was fabricated on Nasicon and alumina substrate using radio frequency (RF) magnetron sputtering. The film was examined using SEM and X-ray diffraction (XRD) after treating the Nasicon-YSZ bi-layer structure at 1300 K for 2 h. The results indicate that a crack free YSZ film was produced on Nasicon surface that was well bonded to the substrate. The conductivity of sputtered YSZ thin film measured by ac-impedance spectroscopy has been found to be higher than that of YSZ pellet by approximately half an order of magniture. The bi-electrolyte planar sensor displays rapid response (t(95) <= 200 s) to CO2 compared to the tube type sensor (t(95) <= 700 s) and the measured open circuit voltage of the electrochemical CO2 sensor has been found to be Nernstian at all temperatures. (C) 2005 Springer Science + Business Media, Inc.