The photoreaction converting the syn form of the imino tautomer of 1-methyl-N-4-hydroxycytosine into the anti form was observed for the compound isolated in argon, nitrogen, and xenon low-temperature matrixes. It was found that the reaction is reversible and that it leads to a photostationary point. The assignment of the form initially present in the matrix to the syn isomer and of the photoproduct to the anti isomer was based on the good agreement between the experimental IR spectra and the spectra theoretically simulated at the density functional theory (B3LYP/6-31G**) level.