Infrared spectra of N⁴-hydroxycytosine isolated in argon and nitrogen low-temperature matrixes are reported. The compound was found to adopt the syn structure of the imino-oxo tautomeric form exclusively. A photoreaction (λ > 250 nm) converting this form into the anti isomer was observed. The reaction is reversible and leads to a photostationary point. The initial infrared spectrum and the spectrum of the photoproduct were assigned to the syn and anti isomers, respectively. This assignment is based on a close agreement between the experimental spectra and the spectra theoretically simulated at the DFT(B3LYP)/6-31 G** level of theory.