The electrical properties of grain boundaries are analysed by admittance spectroscopy methods in order to design gas sensors by enhancing the activity of surface states in the detecting operation. A double Schottky barrier explains the grain boundary action under the presence of surrounding gases. The height of this barrier is a function of gas concentration due to the trapping of excess charge generated by gas adsorption at the interface. The admittance of the sample is then modified by these charges. A plot of the real and imaginary components of the admittance versus frequency characteristic gives information about the different parameters that play a role in the conduction mechanisms, i.e., trapping centres, Schottky barriers, etc. The analysis of this plot for several gases (CO and NO,) at different concentrations and temperature allows us to determine the nature of the detection mechanism and the behaviour of the barriers. All these parameters and their influence in the detection of gases are used to design a sensing element suitable for the proper gas atmosphere. These methods have been applied to the design of a CO sensor based on tin oxide films for domestic purposes, the sensitivity of which is presented.