The electronic structures and relative stabilities of N-substituted benzotriazoles (BtR) have been studied by UV photoelectron spectroscopy (UPS) and high-level ab initio methods. We show that the UPS spectrum of 1H-benzotriazole corresponds to a mixture of tautomers. The N-substituent effects in benzotriazoles were shown to depend on the mode of attachment of the substituent to the Bt residue. We have estimated the conjugation effects between benzene and triazole rings in N-substituted benzotriazoles by using isodesmic reactions and high-level composite ab initio methods. The 2H-benzotriazoles (Bt2R) appear to be stabilized upon ring conjugation while on the other hand the 1H-benzotriazoles (Bt1R) are destabilized. The electronic structure of benzotriazoles can be related to photocycloaddition reactions and to regioselectivity exhibited during nitration.