The prototropic tautomerism in anhydrous DMSO of benzotriazole and six derivatives symmetrically substituted on the benzene ring (5,6-dichloro, tetrachloro, 4,7-dibromo, tetrabromo, 5,6-dimethyl, and tetramethyl), was followed by both experimental (C-13 NMR and UV spectroscopy) and theoretical methods. In all of the analyzed systems, predominance of the asymmetric form, N(1)/N(3) protonated, was found. The rates of the N(1)-H <-> N(3)-H prototropic equilibrium, estimated by C-13 NMR techniques, were in the medium exchange regime of 300-3000 s(-1), and are correlated with the spectroscopically determined pK(a) values in aqueous medium, and the anionic forms are the putative rate-limiting intermediate states.