- The vibronic spectrum of laser desorbed and jet cooled cytosine consists of bands from two major tautomers (keto and enol) as revealed by UV-UV and IR-UV double resonance spectroscopy and methyl blocking experiments. Only one isomer each was observed for the cytosine dimer and for the cytosine - 1-methylcytosine mixed dimer. These isomers form CO⋯HNH/NH⋯N hydrogen bonds. Cytosine - 5-methylcytosine exhibits three isomers: one again with CO⋯HNH/NH⋯N connectivity, the second with CO⋯HNH/NH⋯N interaction but one cytosine in the enol form and the third with symmetrical CO⋯NH/NH⋯OC bonds. These are also the most stable clusters according to molecular dynamics/quenching and ab initio quantum chemical calculations. The experimental IR spectra of these isomers agree well with the calculated normal mode vibrational spectra. The vibronic spectra of the clusters are blue shifted relative to the monomer spectra by more than 1000 cm−1 indicating a considerable reduction of dimer stability upon electronic excitation.