Dithiooxamide [rubeanic acid, NH2C(=S)C(=S)NH2)] monomers were studied by FTIR spectroscopy combined with the low-temperature matrix-isolation technique. The most stable dithione-diamino tautomer of the compound was exclusively observed in argon matrixes immediately after deposition. Upon UV (lambda > 345 nm) irradiation the dithione-diamino form transformed, by a double-proton transfer, into the dithiol-diimino tautomer. Theoretically supported analysis of the infrared spectrum emerging upon UV irradiation allowed identification of one of the conformers of the dithiol-diimino tautomer as the dominating photoproduct. Smaller quantities of other conformers of the dithiol-diimino tautomer were also found to be photogenerated. For the UV-irradiated matrix kept subsequently at 10 K and in darkness, a dithiol-diimino-->dithione-diamino tautomerization leading to partial recovery of the initial form of the compound was observed. The only possible mechanism of this ground-state transformation at cryogenic temperature is synchronous double-proton tunneling. The experimentally obtained time constant of this process was 18 h.