6,9-Di(tert-butyl)-1-methyltetrazolo[1,5-a]perimidine (1) has been synthesized from naphthalene in seven steps. The EPR spectra, recorded after irradiation of 1 in a butyronitrile matrix at 77 K (lambda = 351 nm) and in Ar and Xe matrixes at 4.6 K (lambda > or = 345 nm), showed a six-line, high-field signal (Delta m(S) = +/- 1), centered at 3350 G in butyronitrile, along with a half-field signal (Delta m(S) = +/- 2), which is characteristic for triplets. Simulation of the observed EPR spectra gave values for the zero-field splitting parameters of |D/hc|/cm(-1) = 0.0105, |E/hc|/cm(-1) = 0.0014 in butyronitrile and |D/hc|/cm(-1) = 0.0107, |E/hc|/cm(-1) = 0.0016 in Ar. These EPR parameters are consistent with the diradical 5,8-di(tert-butyl)-2-(N-methylimino)perimidine-1,3-diyl ((3)2) as source of the EPR spectra. Linearity of the Curie-Weiss plot and UB3LYP and (14/14)CASPT2 calculations of the singlet-triplet energy difference (DeltaE(ST) approximately 8-10 kcal/mol) indicate that the triplet is the ground state of 2, as predicted for such a nondisjoint diradical.