A DFT-B3LYP/6-31G* level of theory was employed to study the photo- or thermal-induced reactions of 2-nitrophenol (2-NP). Geometrical parameters of 2-NP data and various products were calculated. The molecular structure of 2-NP calculated by the DFT method is similar to the experimental result. Six reaction schemes were given to determine the kind of transitions. Among them, three kinds of transitions were provided, namely proton transfer, internal rotation, and oxygen transfer. Transition structures were calculated by QST3 approach and IRC calculation yielded the potential energy surface. The activation energy of each transition was given. (C) 2003 Elsevier Science B.V. All rights reserved.