To improve the response speed of an electrochemical oxygen sensor, particularly for automotive use, to sense the oxygen content in exhaust gases, and which can be operative either as a polarographic sensor (requiring application of a voltage thereto), or as a potentiometric sensor (having one catalytically active electrode and another which is less catalytically active), an elongated insulating carrier plate (29), for example of a ceramic, has layer electrodes (30, 31) applied to one major surface thereof and a layer of solid electrolyte material (32) over the electrodes, so that the electrodes will be embedded in the solid electrolyte material. The carrier plate (29) is of a material pervious to oxygen molecules. A gas-impervious insulating cover (33), for example of a ceramic glass, covers the layer of solid electrolyte material (32). A heating element (34) protected by a protective cover (35), for example of aluminum oxide, can be applied against the gas-impervious cover layer (33).