A sensor element is described for determining a physical property of a measuring gas, in particular the pressure or the concentration of a gas component in a gas mixture, in particular in the exhaust gas of an internal combustion engine. The sensor element has an electrode exposed to the measuring gas and a reference electrode exposed to a reference gas, ambient air in particular, through a porous volume. The two electrodes are separated from one another by a solid electrolyte. To avoid premature aging of the reference electrode as a result of deposits of foreign substances contained in the reference gas or of chemical interactions caused by the foreign substances, the volume material is selected with respect to its physical and chemical properties in such a way that the foreign substances are bound in the volume and/or are subjected to a chemical reaction.