Ar@C 60 , synthesized by heating primarily C 60 (with trace amounts of C 70 ) under a high pressure of argon gas, was purified by high-performance liquid chromatography (HPLC) and examined by Fourier transform mass spectrometry (FTMS). The sample, which consisted of 0.1% Ar@C 60 , was dissolved in toluene and separated with HPLC using a COSMOSIL 5PYE column as the stationary phase and toluene as the eluent (UV detection). Ar@C 60 was enriched by a factor of about 400 relative to the starting material. Ar@C 60 was also examined by X-ray photoelectron spectroscopy (XPS) but was found to be sensitive to radiation damage and subsequent depletion. Low-energy collisional dissociation of Ar@C 60 + in the FTMS yields primarily C 58 + , generated by loss of the argon atom and C 2 . The ionization potential of Ar@C 60 was bracketed by charge-exchange reactions and was found to be between 7.53 and 7.8 eV, which is indistinguishable from that of C 60 (7.65 eV).