The undoped n-type semiconductor CeO 2 is a less studied but in many respects promising potential new sensor material. As in our former experiments (using different reactive gases) CeO 2 proved to be a highly corrosion-resistant material, in this work detailed investigations have been carried out on its electrical and corrosion behaviour in a wide temperature range (300–800 °C) in oxygen- and SO 2 -containing atmospheres. Our most important results can be summarized as: (a) the resistance of CeO 2 is practically uninfluenced either at 350 or 800 °C by SO 2 if oxygen is present; (b) around these two temperatures no corrosive effect of SO 2 can be observed either; (c) but at 550 °C in the presence of oxygen, the acceptor effect of SO 2 can be observed; (d) at 550 °C during long-term heat treatment (100 h) in an SO 2 - and oxygen-containing atmosphere, the CeO 2 in a thin surface layer transforms into CeO and the same time a reversible incorporation of a few percent of S into this layer can be observed. In the present work the connection between the sensitivity maximum for SO 2 found at 550 °C in the presence of oxygen and a possible surface phase transformation is discussed.