We have calculated equilibrium constants for the reaction of noble gas atoms becoming trapped inside C-60 molecules. Two models of the potential were used : a Lennard-Jones (12,6) potential between the noble gas atom and each carbon atom and the MM3 molecular mechanics program. We assume that the vibration of the noble gas atom is uncoupled to the cage vibrations and calculate the corresponding partition function classically. The results for the two potentials agree for helium and neon but disagree strongly for krypton and xenon, with the MM3 results being much smaller than the Lennard-Jones results.