FT-IR spectra of o-fluoroanisole were measured in a low-temperature Ar matrix with and without UV irradiation. The observed bands. without irradiation, were assigned to the trans conformer with the aid of the quantum chemical calculations at the B3LYP/cc-pVTZ level. By comparing the IR spectrum, after irradiation, with the calculated spectra, the formation of a non-planar conformer was established. The non-planar conformer gradually decayed by a unimolecular process under dark conditions at 16 K, while the trans conformer increased. The back-reaction rate was estimated, and the reaction dynamics between the trans and non-planar conformers was discussed. (c) 2005 Elsevier B.V. All rights reserved.