102 CHEMICAL SENSORS. VOLUME 5: ELECTROCHEMICAL AND OPTICAL SENSORS of the sensors. The reported response time at 500°C varied from 30 s to 5 min for the different sensors. It was claimed that the mixed-potential theory cannot by itself explain the complex behavior of these electrochemical zirconia-based sensors with semiconductor SEs (Chevallier et al. 2008). This can be proven if zirconia-based gas sensors are designed with a large difference in catalytic ac- tivity between the RE and SE in order to obtain high sensitivity to the measured gas. The output EMF of these sensors follows the well-known mixed-potential theory. In this case the linear output signals closely follow the Nernst equation on the logarithmic scale. Having said that, the author must also admit that, if the concentration of measured gas is rather low (less than 50 ppm) or larger that about 500 ppm for NO x measurement, the output EMF deviates from the linear Nernstian line, indicating that, in addition to the competing oxidation/reduction reactions, other reactions and/or conditions at the triple-phase boundary also influence the measuring EMF. Therefore, some authors are reluctant to publish results of their measurements for low and high concentration of measuring gas in order to keep output char- acteristics linear. However, for other zirconia-based gas sensors, designed with no big difference in the catalytic activity between the SE and RE (for example, Nb 2 O 5 SE and Au RE), high sensitivity to the measuring gas cannot be explained only by the mixed-potential theory. The reported negative EMF val- ues for a sensor with a Nb 2 O 5 SE and a Au RE were consistent with the reaction of a reducing gas with oxygen ions adsorbed on the surface of an n-type SE (Chevallier et al. 2008). Consequently, the response of this sensor, in the absence of any catalytic activity, can be properly explained by a semiconducting adsorption­related mechanism and thus by the induced Fermi-level shift.