Based on the fact that the heterojunction and catalytic activity have interesting effects on the detection of CO among the others testing gases (H2 and NO2), further investigations on the gas sensor properties of the nano composite material (Ni0.9Zn0.1/ZnO) were carried out by studying its selectivity, dynamic response and the influence of humidity.To this end and bearing in mind that selectivity is one of the most problematic properties of metal oxide gas sensors, the selectivity of Ni0.9Zn0.1O/ZnO nano composite with respect to CO, NO2 and H2 at different operating temperatures was investigated. Fig. 7 shows the different responses of Ni0.9Zn0.1O/ZnO to the three gases at different temperatures. These results show that at 225◦C the response to 5 ppm of NO2(354%) is higher than that of 300 ppm of CO (33%)and 300 ppm of H2(23%). This implies that this sensor is at least 10.7 times and 15.4 times more sensitive to NO2 than CO and H2,respectively, at 225◦C which is indicative of its good selectivity to NO2. This good selectivity to NO2 can be ascribed to the difference in the optimum operating temperature for each gas. At low temperature (225◦C) NO2 reacts more with the material than CO. In fact NO2 reacts with the sensing material in two ways: directly with free electron (Eq. (1)) and with adsorbed oxygen on the surface (Eq.(2)). However the CO reacts only with the adsorbed oxygen on the surface as shown in Eq. (3). At low temperature owing to the fact that the kinetic of reaction of CO with adsorbed oxygen species is low leading to a low oxygen desorption rate and electron release, a decrease of the response to CO is expected while this phenomenon does not happen for NO2.