Monomers of hypoxanthine isolated in low-temperature Ar matrixes were studied using Fourier transform infrared spectroscopy. Two most stable tautomeric forms of hypoxanthine: oxo-N( 9)-H and oxo-N( 7)-H as well as a very small amount of the minor hydroxy-N( 9)-H tautomer were observed in Ar matrixes directly after their deposition. UV irradiation of the matrixes induced conversion of the oxo-N( 9)-H and oxo-N( 7)-H tautomers of the compound into the hydroxy-N( 9)-H and hydroxy-N( 7)-H forms, respectively. Upon exposure of the matrixes to the UV ( lambda > 270 nm) light, the oxo-N( 9)-H -> hydroxy-N( 9)-H phototautomeric reaction dominated strongly over the oxo-N( 7)-H -> hydroxy-N( 7)-H phototransformation. The latter phototautomeric reaction occurred effectively when matrix-isolated hypoxanthine was irradiated with shorter-wavelength ( I > 230 nm) UV light. Thanks to this wavelength dependency, it was possible to clearly distinguish the oxo -> hydroxy photoreaction within the N( 9)-H tautomers from the analogous phototautomeric process within the N( 7)-H tautomers. All of the observed isomers of hypoxanthine ( substrates and products of the photoreactions) were identified by comparison of their IR spectra with the spectra calculated at the DFT-( B3LYP)/6-31++G( d, p) level of theory.