The infrared spectra of 6-thiopurine (6-mercaptopurine) isolated in Ar and N-2 low-temperature matrices show that the molecule exists as a mixture of thione N(1)H,N(7)H and thiol N(9)H tautomers. The photoreaction converting the thione N(I)H,N(7)H form of the compound into the thiol N(7)H form is observed for the matrix-isolated species. The assignments of the substrate to the thione N(1)H,N(7)H form and the photoproduct to the thiol N(7)H form are confirmed by good agreement between the experimental infrared spectra and the spectra theoretically predicted at the OFT(B3LYP)/6-31G(d,p) level. The thiol N(9)H tautomer is photochemically stable under the conditions of UV irradiation applied in the experiment (lambda > 320 nm). The experimentally estimated ratio of tautomers in the matrix (non-UV-irradiated) [thiol N(9)H]/[thione N(1)H,N(7)H] = 0.24 correlates well with the theoretical estimate of the relative energies of the tautomeric forms of the compound predicted at conventional ab initio calculations [total energies including electronic energies calculated at the Hartree-Fock (HF)/6-31G(d.p) level corrected for electron-correlation contributions at the MP4(SDQ)/6-31G(d,p) level and zero-point energies at the HF/6-31G(d.p) level].