Functional materials for thick film gas sensors have been examined: nanosized powder preparations, such as sol gel and hydrothermal processes, morphological, micro-structural and electrical properties, in particular those depending on average grain dimensions. In this context, a theoretical conduction model able to determine the characteristic length below which a material can be properly considered as nanostructured has been illustrated. The performances of sensing layers, deposited through screen-printing technology, have been examined, also considering their behaviour in on-field applications. In this respect, the influence of some ambient parameters changing with time, such as temperature and humidity, have been investigated. Consequently, accurate NOx and CO concentrations were obtained, compensating the sensors responses from the water vapor effect.