Аннотация:The chemical approaches to improvement of selectivity of semiconductor metal oxide gas sensors are the main subject of this chapter. Current concepts of interrelationships between metal oxide chemical composition, crystal and surface structure and its activity in the reaction with gas phase components are considered. Application of such concepts to the design of sensor materials based on nanocrystalline SnO2 is discussed thoroughly. Experimental data concerning chemical composition, solid–gas chemical interaction activity and sensor properties is given and critically analysed. The possibility of utilization of solid–gas chemical reaction activity concepts for directed synthesis of new metal oxide semiconductor sensor materials with selective response to given gases is highlighted.