To facilitate mass production and permit ready introduction of a substance which provides an oxygen partial pressure reference level, for example the oxygen in ambient air, an elongated plane flat plate-like solid electrolyte body is provided with electrodes thereon, and one of the electrodes is covered by a trough-like cover element to form the reference electrode, the space beneath the trough-like element and the electrode itself being available for the substance which may, but need not be, the oxygen of air, and may be a solid material providing a reference oxygen partial pressure level. The cover plate may, itself, be made of solid electrolyte material and may form a portion of another sensing element, so that a plurality of sensing elements can be superimposed in sandwich-like fashion. A heater element can be applied to a flat plate element of the assembly where desired. The sensors can operate in the potentiometric or polarographic mode.