N²-hydroxyisocytosine and 1-methyl-N²-hydroxyisocytosine were studied using a matrix isolation technique combined with infrared absorption spectroscopy. For N²-hydroxyisocytosine isolated in an Ar matrix (at 10 K), two imino–oxo isomers, one with the hydroxyimino =N–OH group directed toward the N1–H group (the form called further anti) and the second with the =N–OH group directed toward N3–H (syn), were observed in the ratio 1.4:1. The syn isomer is converted totally to the anti form after UV (λ > 295 nm) irradiation of the matrix. A small amount of the N(3)H-hydroxy–amino tautomer of N²-hydroxyisocytosine was also detected in the matrix. This form did not react photochemically. For 1-methyl-N²-hydroxyisocytosine, only the syn form of the imino–oxo tautomer was observed after deposition of the matrix. UV (λ > 295 nm) irradiation induced a photoreaction converting this isomer into the anti form. After 15% of the starting material had been converted into the product, a photostationary state was achieved, and no further progress of the reaction was observed. Subsequent UV irradiation (λ > 335 nm) caused a back reaction, leading to a disappearance of the anti form and to the recovery of the initial syn isomer. All isomers were identified by comparing their experimental IR spectra with the spectra theoretically calculated at the DFT(B3LYP)/6-31G(d,p) level, where DFT is the density functional theory. Good agreement between the observed and predicted patterns of the spectral lines allowed for reliable identification. The experimental IR spectra were interpreted and discussed. The relative energies of the 12 isomers of N²-hydroxyisocytosine were calculated at the MP2/6-31G(d,p) and MP4//MP2/6-31G(d,p) levels. For six isomers of 1-methyl-N²-hydroxyisocytosine, the calculations were carried out at the MP2/6-31G(d,p) level. The anti form of the imino–oxo tautomer of N²-hydroxyisocytosine and the syn form of the imino–oxo tautomer of 1-methyl-N²-hydroxyisocytosine were predicted to be the most stable.