Infrared spectra of naphthalene in the lowest triplet state T-1 were measured by matrix-isolation FTIR spectroscopy. From analyses of the difference between the spectra measured before, during, and after UV irradiation, 10 infrared bands have been assigned to the vibrational modes of the T-1 state for the normal species and four bands to those of the fully deuterated species. The optimized geometry and harmonic force constants of the T-1 state were obtained from an ab initio calculation at the CIS/6-31G* level. The calculated vibrational frequencies agree well with those in the observed matrix-isolation spectra. (C) 1999 Elsevier Science B.V. All rights reserved.