Two types of exhaust sensors have been studied from the point of view of applying them in feedback systems to control the air/fuel (A/F) operating point of automotive engines. The particular sensors studied were zirconia (ZrO2) sensors and titania (TiO2) sensors. Both sensors were without built-in heaters and designed to indicate when engine A/F passes through stoichiometry. The paper includes results obtained from engine dynamometers and test vehicles; but the major experimental results are from laboratory tests specifically designed to establish advantages or limitations of a specific sensor and its associated circuitry when used in engine A/F control systems.Calibration data were measured for the zirconia sensors in order to plot curves of sensor output voltage versus A/F with temperature (T) as a parameter. The zirconia sensor was found to be especially sensitive to the presence of hydrogen (H2) in exhaust gas. Data establishing this sensitivity are presented. Transient response capabilites of the zirconia sensor are explored and experimental results pointing to a structural transformation in the sensor are presented.Experiments similar to those for the zirconia sensors were carried out for titania sensors. Calibration data for resistance as a function of A/F with T as a parameter are presented. Two effects observed in titania sensors in laboratory tests, transition-through-stoichiometry hysteresis and rich-end-point drift, are described, along with some discussion of transient response capabilities.Since both types of sensors and their associated circuitry are in a continuing process of development, basic importance is attached to the measurement techniques and the general types of information developed rather than to the experimental results obtained for either particular sensor at the present time.