A measuring sensor is described for determining a physical property of a measured gas, especially for determining the oxygen concentration or the pollutant concentration in the exhaust gas of internal combustion engines, which has a sensor element that is exposable to the measured gas which is at least partially coated with a protective layer that protects against harmful components in the measured gas. In order to achieve producing a “contamination protection”, that is cost-effective from a manufacturing technology point of view, particularly against silicon compounds and phosphorus compounds, the protective layer (26) is made of highly active γ- or δ-aluminum oxide (Al2O3) having additives of compounds of the alkaline metals group, the alkaline earths group, the IV B subgroup or the lanthanides group.