The stability of the azafullerenes of the type(C59N)Xwas studied by using IR, mass spectroscopy, optical and Raman experiments. X represents either hydrogen or an other cage ofC59N.Experimental data indicate thermal stability of the solid phase of the dimer up to 600 K followed by very slow degradation extending beyond 700 K. This is compared to our latest results on the temperature stability ofC59HNwhich was found to be stable only up to 540 K. A sudden change in the spectra at this temperature gives evidence for a transition to a new air stable phase. Since the mass spectra show no loss of hydrogen the new phase is claimed to be polymerizedC59HN.When heated in vacuum to about 700 K this phase dimerizes into(C59N)2.©2001 American Institute of Physics.