It was found that 2,6-dithiopurine isolated in low-temperature argon and nitrogen matrixes adopts only one tautomeric form: dithione-N7H. The UV (lambda > 345 nm) induced reactions converting this tautomer into dithiol-N3H and dithiol-N7H isomers were observed in Ar and N-2 matrixes. The first of these reactions represents a novel type of photoinduced double-proton transfer, whereas the second is analogous to that previously observed for 2,4-dithiouracil (Lapinski, L.; Nowak, M. J.; Kolos, R.; Kwiatkowski, J. S.; Leszczynski, J. Spectrochim. Acta A 1998, 54, 685). The assignment of the form initially present in the matrix to the dithione-N7H isomer and of the photoproducts to the dithiol-N3H and dithiol-N7H isomers was based on the good agreement between the experimental IR matrix spectra and the spectra theoretically simulated at DFT(B3LYP)/6-31G(d,p) level.