In this paper, we present the first direct evidence for the formation of endohedral fullerenes from prompt gamma emission of neutron irradiated Xe held interstitially in the C-60 lattice. Following neutron activation, solutions of the irradiated solid were analyzed by high-pressure liquid chromatography and gamma spectroscopy which showed Xe radioisotopes to be present in fractions that eluted at longer times relative to untreated C-60. The observation of Xe radioisotopes at these articular times was attributed to elution of compounds based on Xe@C-60. Additional support for this conclusion was obtained from isotopic ratios measured by gamma spectroscopy. The present results provide convincing evidence in support of the gamma recoil mechanism for trapping radionuclides in fullerene cages which had previously been inferred from indirect evidence.