Fig. 6 shows the response variation as a function of operating temperature (in the range 300–500 ◦C) for different CO concentrations. It shows that CO concentration as small as 500ppm could be easily detected. The inset of Fig. 6 shows the values of the response at 390 ◦C (that is maxima in response versus temperature curve) as a function of the CO concentration. The observed linear behavior in the logarithmic scale is an indication that the current-percolation model [24] holds true for the CeO2 thin films. Ferroni et al. [25] observed a similar linear behavior in logarithmic scale for MoO3 films. It is clearly evident from Fig. 6 that pure CeO2 thin film gas sensors were highly sensitive to CO. Optical, XPS and AFM studies of these films revealed that the prepared CeO2 films were highly porous. Thus, the large response observed with CeO2 could be attributed to the porosity of the films. Park and Mackenzie [26], while studying the alcohol sensing properties of tin oxide thin films, concluded that the sensing properties strongly depend on the film porosity rather than thickness or grain size. There fore, it can be concluded that the CO sensing properties of CeO2 sensors strongly depend on and increase with the porosity.