The IR spectra of thione and thiol tautomers of 2-quinolinethione isolated in Ar and N 2 low-temperature matrices are reported. The UV-induced photochemical reaction, transforming the initially abundant thione form into the thiol form, was used for separation of the spectra of the two tautomers. The ratio of the tautomers before UV irradiation of the sample was estimated as [thione]/[thiol] = 7:1. The experimental spectra of 2(1H)-quinolinethione and 2-quinolinethiol were interpreted by comparison with the spectra calculated theoretically at the ab initio SCF/6-31G * * level.