Among the many physical and chemical strategies used to make air pollution gas sensors for CO x , NO x , SO x monitoring, those employing electrochemical detection means offer the highest selectivity, long lifetimes, low drift and low costs of manufacture. Targeting the three gases CO 2, NO 2 and SO 2, new solid-electrolyte-based, selective potentiometric gas sensors were fabricated in both bulk and thin film versions. For all six bulk and thin film sensors, the EMF responses were related to the analyzed target gas pressure by the Nernst law. For each of the target gases CO 2, NO 2 and SO 2, our thin film micro-sensor exhibits excellent gas-sensing sensitivity, response time, selectivity and recovery compared to the bulk sensor. The carbon dioxide micro-sensor characteristics obtained at 250°C were 45±3 mV/decade for the sensitivity, 10 to 30 s for the response time and 60 s for recovery time. The nitrogen dioxide micro-sensor characteristics obtained at 200°C were 48±3 mV/decade for the sensitivity, 2 to 10 s for the response time and 10 s for recovery time. The sulfur dioxide micro-sensor characteristics obtained at 250°C were 50±3 mV/decade for the sensitivity, 1 to 2 s for the response time and 5 s for recovery time. In comparison, the bulk sensor CO 2 characteristics obtained at 250°C were 50±3 mV/decade for the sensitivity, 60 to 300 s for the response time and 900 s for recovery time. To verify selectivity in air samples, the sensitivity of each gas detector with respect to the gases CO 2, NO 2 and SO 2, oxygen, CO, and humidity were evaluated. The CO 2 sensor was insensitive to the later three gases, but there was a measurable EMF drift produced for NO 2 at concentrations above 10 ppm and for SO 2 at concentrations above 80 ppm. Similarly, the NO 2 sensor operating at 200°C was insensitive to all other gases except for SO 2 at concentrations above 80 ppm. Finally, the SO 2 sensor operating at 250°C was insensitive to all other. We optimized our processing to obtain a dense strongly adhering deposited solid electrolyte thin film. XPS and XRD studies show that the chemical composition and structure of the deposited solid electrolyte thin-film and targets for RF magnetron sputtering were the same to within experimental error. For brevity, complete results are presented only for the CO 2 micro-sensor. The results for the other two gas micro-sensors are essentially the same and will be published elsewhere.