From these results, we can conclude that the gas sensor material based on Ni0.9Zn0.1O/ZnO shows a good selectivity at 225◦C for NO2 and an increased response to CO at 300◦C.Fig. 8 shows the dynamic and reproducible response of the Ni0.9Zn0.1O/ZnO gas sensor exposed to different concentrations of CO at 300◦C. As can be seen, the sensor resistance reaches an equilibrium value upon gas exposure and settles back to the original value when the test gas is vented and this indicates a good reproducibility of the sensor. The dependence of the sensor response on the concentration of CO was also noticed showing that the response time is limited by the reaction rate of CO with adsorbed oxygen species. The response of the sensor is 135, 183 and 222 for 100, 200 and 300 ppm of CO respectively. The results indicate that the gas response increases with increasing concentration of CO from 100 to 300 ppm. The response and recovery times are calculated to be 59 s and 370 s respectively.