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A new disposable radio-frequency identification (RFID) sensor for detecting oxygen in packages with a protective atmosphere is presented. For safety reasons and system costs in consumer packages, no battery or energy harvesting devices can be used. Each part of a package, especially in food packaging, must be completely safe even if it is swallowed. Several materials have been investigated that safely react with oxygen and thus change electrical parameters without the need of an additional energy supply. In particular linseed oil was tested, because it is known to react in oxygen-containing atmosphere from liquid to solid. Linseed oil is used not only as food but also as a key part in ecological paint coatings. A significant relative change of capacity was observed during linseed oil drying, which results in −20% after 5 h and −38% after 30 h at an oxygen concentration of 20.5 and 50% relative humidity, respectively. Pure unsaturated fatty acids were also tested in an oxygen-containing atmosphere and showed similar behaviour. The reaction speed is partially dependent on the level of unsaturation of fatty acids. The oxygen sensor is coupled with an RFID front end with an internal charge time measurement unit for capacity determination. The combination of sensor element, sensitive material and RFID allows for biocompatible and save systems that indicate the presence of oxygen within a package.