A porous Al.sub.2 O.sub.3 thick film covers the SrTiO.sub.3 layer of an oxygen sensor, said SrTiO.sub.3 layer being contacted by means of two Pt electrodes and deposited on an Al.sub.2 O.sub.3 substrate. The electrically insulating Al.sub.2 O.sub.3 layer bears a protective layer, which is exposed to the exhaust gas and is preferably also made of SrTiO.sub.3. This construction ensures that the output signal of the sensor representing the oxygen partial pressure then depends only on the resistance or, respectively, conductivity value of the non-contaminated SrTiO.sub.2 sensor layer.