A substantial amount of endohedral fullerene, Ar@C60 (>1 mg), was successfully obtained by heat treatment of C60 powder under high pressure Ar atmosphere, followed by sequential HPLC separation. The isolation of pure Ar@C60 was demonstrated by TOF-MS, 13C NMR spectroscopy and powder X-ray diffraction. We report here the synthesis of the first endohedral fullerene superconductors, K3(Ar@C60) and Rb3(Ar@C60) having critical temperatures of about 18 and 27 K, respectively. The decrease of Tc (ΔTc) in both K3(Ar@C60) and Rb3(Ar@C60) compared with pristine compounds (ΔTc >1 K and >2 K, respectively) is rather large, implying that the influence of encapsulated Ar atom on the C60 cage and therefore to the superconductivity of its derivatives, is more pronounced than expected. Indeed, synchrotron X-ray powder diffraction results showed, rather unexpectedly, that Ar@C60 and C60 have different unit cell parameters therefore different van der Waals radii.