Abstract For the first time the lowest excited triplet state T 1 of BNC-heterocycles has been investigated. The phosphorescence and ODMR-spectroscopic measurements allow conclusions with respect to the distribution and conjugation of the π-electrons and thus the controversial question of the aromaticity of these compounds. Whereas T 1 in 2-methyl-1,3,2-benzodiazaborolidine may be described by a benzene-like 3B 1u state which is distorted along the ν 8 vibration the decay characteristics of the triplet state of 2-methyl-1,3,2-benzothiazaborolidine is dominated by the local C 2v symmetry around the heavy atom sulfur. In the case of 2-methyl-2-boradihydroperimidine we find a naphthalene-like 3A 1 state for T 1. The zero-field splitting confirms that the electronic triplet excitation is essentially localized in the benzene or naphthalene nucleus and the annellated heterochain participates only little in the π-conjugation.