Charge transfer is a general phenomenon observed for all endohedral mono-metallofullerenes. Since the detection of the first endohedral metallofullerene (EMF), La@C₈₂, in 1991, it has always been observed that the oxidation state of a given encapsulated metal is always the same, regardless of the cage size. No crystallographic data exist for any early actinide endohedrals and little is known about the oxidation states for the few compounds that have been reported. Here we report the X-ray structures of three uranium metallofullerenes, U@D_3h-C₇₄, U@C₂(5)-C₈₂ and U@C_2v(9)-C₈₂, and provide theoretical evidence for cage isomer dependent charge transfer states for U. Results from DFT calculations show that U@D_3h-C₇₄ and U@C₂(5)-C₈₂ have tetravalent electronic configurations corresponding to U⁴⁺@D_3h-C₇₄ ⁴⁻ and U⁴⁺@C₂(5)-C₈₂ ⁴⁻. Surprisingly, the isomeric U@C_2v(9)-C₈₂ has a trivalent electronic configuration corresponding to U³⁺@C_2v(9)-C₈₂ ³⁻. These are the first X-ray crystallographic structures of uranium EMFs and this is first observation of metal oxidation state dependence on carbon cage isomerism for mono-EMFs.