Abstract: Tin oxide doped with magnesium oxide or niobium pentoxide and prepared as a sintered material is found to be highly sensitive to oxygen partial pressure both in the presence of free oxygen and in the presence of oxidizable gases, such as carbon monoxide or the lower oxides of nitrogen, and even in the presence of mixtures of both, such as in the exhaust gas of an internal combustion engine. The electrical conductivity varies by substantially more than an order of magnitude in response to a change in the oxygen partial pressure of about one-half an order of magnitude. The doped oxides operate as poor catalysts for the oxidation of oxidizable components of the exhaust gases. Chromium oxide doped with tin oxide shows the effect of an excessively good catalysis from the point of view of range of measurement, but is usable to give a sharp indication of the appearance or disappearance of free oxygen in a gas mixture.