Chemical sensing on oxide sensors is a complex phenomenon involving catalytic activity as well as electronic properties. Thus, the properties of oxide sensors are highly sensitive towards structural changes. Effects like surface area, grain size, and, in addition, the occurrence of defects give separate contributions to the current. Structure-property-function relationships can be elucidated using a combination of state-of-the-art analytical techniques. It is shown, that impurity atoms in the oxide lattice influence the performance of ZnO sensors more strongly than the other factors.