Thiourea monomers isolated in a low-temperature argon matrix adopt exclusively the thione tautomeric form. Upon UV (lambda > 300 nm) irradiation a photoreaction converting the initial isomer of the compound into its thiol tautomer occurred. Two structures of the photoproduct (with syn and anti orientation of the imino C=N-H group with respect to the C-S bond) were identified in the matrix. Subsequently, a back reaction transforming the thiol form into the initial thione tautomer was observed for the matrix kept at 10 K and in darkness. The molecules with the anti conformation underwent this process, whereas those in the syn form remained unchanged. The only possible mechanism of the ground-state thiol --> thione transformation at low temperature is proton-tunneling through the very high energy barrier of 9030 cm(-1) (108 kJ mol(-1)) (calculated at the MP2/6-31++G(d,p) level). The experimentally obtained time constant of this process was 52 h. The structure of the photoproduced species was identified by comparing the experimental IR spectra with the theoretically calculated spectra obtained at the DFT(B3LYP)/6-31++G(d,p) level. UV-induced thione --> thiol reaction of thiourea as well as the ground-state thiol --> thione proton tunneling in the absence of light are reported for the first time.