The stable, two-atomic radical NO, which is an important biophysical messenger, was studied with magnetic resonance methods. Caused by its specific electronic configuration, it has sensor properties: its magnetic moment sensitively depends on its environment and restricted mobility therein. The interaction with solvents or solid phases can thus be quantified. Encapsulating the radical in the nearly spherical cage of modified C₆₀ fullerenes is a perfect situation to study this effect.