Two thione conformers of monomeric thiosemicarbazide were trapped from the gas phase into a low-temperature Ar matrix. A phototransformation converting the less stable form of the compound into the most stable conformer was induced by irradiation with near-IR (lambda = 1462 nm) or UV (lambda > 320 nm) light. This photoeffect allowed separation of the IR spectra of the observed thione forms. The structures of both observed isomers were identified by comparison of the separated experimental IR spectra with the spectra theoretically predicted for two most stable forms of the compound. The population ratio of the two conformers in an Ar matrix, prior to any irradiation, was estimated to be equal approximate to 2:1. Irradiation of matrix-isolated thiosemicarbazide with shorter-wavelength UV (lambda > 270 nm) light induced a phototautomeric reaction generating thiol forms of the compound.