Theoretical data reveal that benzotriazole in its first triplet electronic state, T-1, exhibits two energy minima. One is of C-s symmetry, corresponds to the 1H tautomer which has its 0-0 component at 398 nm; the other lacks symmetry, corresponds to the 211 tautomer which has its 0-0 component at 473 rim. The 2H tautomer is intrinsically more stable (by ca. 11 kcal/mol). Consistent with the previous theoretical inferences, experimental evidence for polycrystalline benzotriazole at 77 K, which consists of 100% of the 1H tautomer in the ground state, only exhibits the phosphorescence for the 2H tautomer. (c) 2005 Elsevier B.V. All rights reserved.