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Electron spin resonance (ESR) spectroscopy is used in this study in connection with the mass spectrometry one of the most promising method to characterize the state of endohedral fullerenes. The aim of this contribution is to study scandium, yttrium, lanthanum, cerium, praseodymium, samarium, europium, holmium, thulium and lutetium containing fullerenes in dependence on the production and handling conditions with respect to their electronic structure. For the most metallofullerenes both single filled and double filled fullerenes were measured. The mass spectrometric signals of single filled fullerenes are much larger than the signals of double and higher filled metallofullerenes. ESR spectroscopic properties were followed especially in solution of isolated metallofullerenes of high purity characterized by well resolved ESR spectra but also of the solid soot extract resulting in low resolved ESR spectra. In this way new information on the electronic states and the influence of oxygen on endohedral metallofullerene were obtained. The interaction of the lanthanofullerence with oxygen is studied in detail. Different stabilities in different solvents in the presence of oxygen were found for La at C82 molecules. For scandium, yttrium and lanthanum containing fullerenes ESR spectra with hyperfine structure splittings corresponding to the nuclear spin of the metal ion were found. Experiments with lanthanoides results in fullerenes, showing mass spectra with lines equivalent to the corresponding endohedral compounds with single and double filed cages, but no ESR spectra due to the interaction of an unpaired electron with the nuclear spin of the lanthanoide ion could be detected in the magnetic field range of 0 to 800 mT.
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