CO2 and CO sensing properties of an electrochemical gas sensor employing an anion-conducting polymer (ACP) membrane as an electrolyte and Pt only, Pt mixed with ACP or 10 wt% Pt-loaded carbon black (Pt/CB) mixed with ACP as an electrode material, which were denoted as an EC(Pt), EC(Pt-ACP) or EC(Pt/CB-ACP) sensor, respectively, have been investigated in this study. All sensors showed positive Nernstian response to 400-20,000 ppm CO2 in humid air at 30 degrees C, and the EC(Pt-ACP) sensor showed large CO2 response in comparison with those of EC(Pt) and EC(Pt-CB/ACP) sensors. These CO2-sensing properties were largely dependent on humidity in sample and reference gases and CO2 concentration in the reference gas. In addition, the electromotive force (EMF) of the sensors also varied with a change in O-2 concentration in a CO2-free sample gas, when CO2-free air was used also as a reference gas. However, the EMF was little dependent on the O-2 concentration in the sample gas containing CO2. From these results, a 2-electron O-2-free redox reaction of CO2 may determine the potential of the sensing electrode under atmosphere containing CO2. On the other hand, EC(Pt-ACP) and EC(Pt/CB-ACP) sensors showed slow and negative response to CO in humid air at 21 degrees C, and the magnitude of CO response of these sensors was almost proportional to the logarithm of CO concentration. In addition, the EMF was largely dependent on the O-2 concentration in the sample gas containing CO. The O-2 response behavior and current-voltage characteristics in N-2 containing 1000 ppm CO or 20% O-2 confirmed that the mixed potential consisting of CO oxidation and O-2 reduction is the most important factor to determine the magnitude of CO response. (c) 2012 Elsevier Ltd. All rights reserved.