Abstract A step-by-step experimental and ab initio SCF/6-31++ G** procedure is described for the interpretation of the matrix FT-IR spectra of H-bonded complexes of cytosine or isocytosine with water. The method involves a detailed study of several compounds, each modelling one or a few cytosine sites and/or tautomers in order of increasing complexity. Results are presented for tautomerization processes in 2-OH-pyridine, 4-OH-pyrimidine ...