Dioxygen is one of the key molecules on Earth. It plays an essential role in the atmosphere, the hydrosphere, the geo- sphere and not the least the biosphere.(1) Many physiological transformations, chemical reactions, and (bio)technological processes produce or consume O2, while anoxic species, numerous chemical syntheses, and manufacturing protocols demand its complete absence. Its extraterrestrial presence hints at the potential presence of life in the form we know.(2) Trace oxygen detection is also important in aerospace research(3) and from a safety standpoint,(4) as oxygen leaks can cause fires and explosions and can be harmful in storage chambers and packaged food.(5) Common trace oxygen sensors are based on amperometry (Clark electrodes). These instruments are sensitive and applicable over a wide temperature range but are difficult to miniaturize, invasive, and limited to discrete points.(6) Optical sensors overcome these limitations. Most are based on the quenching of the long-lived luminescence exhibited by polycyclic aromatic hydrocarbons, transition-metal com- plexes, and metalloporphyrins.(7) These compounds are typi- cally placed in inert polymer membranes. Highly permeable matrices are employed in order to sense traces of O2. (8) Herein we show that an as yet unmatched sensitivity combined with an unmatched brightness at high temperatures can be achieved by exploiting the extremely efficient quenching of the delayed fluorescence of the ellipsoidal fullerene C70