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A solid potentiometric CO2 sensor has been developed using Li3PO4 as the electrolyte, Li2TiO3/TiO2 as the reference electrode, and Li2CO3 as the sensing electrode. The Li3PO4 film was deposited on Al2O3 substrate by resistance heating evaporation. Two Pt electrodes with designed patterns were formed on the electrolyte film by micro fabrication technologies. The reference electrode and the sensing electrode were prepared by thick-film technology to cover part of the two Pt electrodes. Various properties were tested: sensitivity under different conditions, response and recovery properties, cross-interference performance, and stability. The values of ΔEMF/dec are 22 mV/dec, 45 mV/dec, and 78 mV/dec in accordance with the operating temperatures: 300°C, 400°C, and 500°C. At 500°C, the response time and recovery time are 30 s and 40 s, respectively, which is better than those of 400°C. When the thickness of Li3PO4 film is greater than 650 nm, the EMF output of the sensor changes little with change of the film thickness. From the results, it can be concluded that the optimal working temperature for the sensor is 500°C, and the optimal thickness of Li3PO4 is about 650 nm.