Solutions containing ³He@C₆₀, ¹²⁹Xe@C₆₀, and varying amounts of 9,10-dimethylanthracene (DMA) were allowed to reach equilibrium, and the ³He and ¹²⁹Xe NMR spectra were taken at the same temperature. Each spectrum showed peaks for the unreacted X@C₆₀ and for the monoadduct. The ratios of the peak heights show that the included xenon atom substantially changes the equilibrium constant. This change is temperature dependent, meaning that the xenon atom changes both ΔH and ΔS for the reaction. DMA is more reactive with He@C₆₀ at low temperatures and with Xe@C₆₀ at higher temperatures. The difference in chemical shift between the monoadduct and the unreacted X@C₆₀ is more than twice as large for Xe than for He and in the opposite direction. Calculations show that the electron density in Xe@C₆₀ is higher than that in empty C₆₀ on the outside of the cage.