Matrix-isolation infrared spectroscopy was used to identify a reaction intermediate of 2-nitrophenol with an aid of density-functionaltheory (DFT) calculations. The observed FTIR bands of a product from 2-nitrophenol upon irradiation of 248 nm were assigned to an aci-nitro isomer among 2 x 2 possible conformations, cis/trans around the C=N+(O-)OH and CN+ (O-)-OH bond axes, by comparison with calculated spectral patterns. A reverse reaction from aci-vitro isomer to 2-nitrophenol was observed upon irradiation with lambda > 450 nm. The mechanism of this reversible photoreaction is discussed in terms of the vertical transition wavelengths and the oscillator strengths obtained by time-dependent DFT calculation. (c) 2006 Elsevier B.V. All rights reserved.