Abstract This work deals with a theoretical study of the interaction between the amino-oxo (a-o) and cis amino-hydroxy (a-h) tautomers of cytosine with one hydrogen peroxide molecule (HP). The optimized geometries, binding energies and harmonic vibrational frequencies are calculated using DFT/B3LYP functional combined with the 6-31++G(d,p) basis set. The results of a NBO analysis are reported as well. Four stable a-o–HP and six stable a-h–HP complexes are found on the potential energy surface. The structures are cyclic, some of them being stabilized by weak intermolecular CH…O interactions. The binding energies range from −12.9 to −47.7 kJ mol −1. HP binds to a-o cytosine 5.4–7.9 kJ mol −1 stronger than to the enol form. The data are compared with uracil–H 2O and uracil–HP complexes. The binding energies corresponding to the formation of C O…HO…HN hydrogen bonds depend exponentially on the proton affinities of the O atoms of the carbonyl and hydroxyl groups and on the deprotonation enthalpies of the OH and NH bonds. The frequencies of the inversion mode of the NH 2 group are very sensitive to the pyramidal character of this group. The ν(CH) stretching vibrations of the CH bonds involved in the interaction show the characteristic features of the blue-shifted hydrogen bonds. The frequency shifts of the ν(OH) vibration of HP and of the ν(CH) vibration of cytosine are discussed in terms of the occupation of the corresponding antibonding orbitals and the rehybridization occurring upon complex formation.