- In this paper we present detailed R2PI spectra with IR–UV and UV–UV double resonance measurements of the guanine dimer (GG) and its methylated derivatives. We show that there are two isomers of GG in the investigated wavelength range from 32565 to 33600 cm−1. We were able to assign the two isomers to specific structures based on comparison of the intermolecular vibronic patterns of the dimers with and without methylation, on analysis of the IR spectra in the range of the OH and NH stretching vibrations and on comparison with ab initio calculated dimer stabilities and vibrational frequencies. In both structures both guanine moieties are in the keto tautomeric form, even though the enol tautomers are also present in the beam. One isomer exhibits nonsymmetric hydrogen bonding with HNH⋯N, NH⋯N and CO⋯HNH interactions (K9K7-2). The other isomer has a symmetrical hydrogen bond arrangement with CO⋯NH/NH⋯OC bonding (K9K7-1). The most stable guanine dimer forms CO⋯NH/NH⋯OC hydrogen bonds and has C2h symmetry (K9K9-1). Due to its strong exciton splitting the allowed S0–S2 transition is outside the investigated spectral range. We did not observe any keto–enol or enol–enol dimers in the investigated wavelength region. The calculations predict these dimers to be considerably less stable.