The nucleobases uracil (U) and thymine (T) offer three hydrogen-bonding sites for double H-bond formation via neighboring N-H and C=O groups, giving rise to the Watson-Crick, wobble and sugar-edge hydrogen bond isomers. We probe the hydrogen bond properties of all three sites by forming hydrogen bonded dimers of U, 1-methyluracil (1MU), 3-methyluracil (3MU), and T with 2-pyridone (2PY). The mass- and isomer-specific S-1 <- S-0 vibronic spectra of 2PY center dot U, 2PY center dot 3MU, 2PY center dot 1MU, and 2PY center dot T were measured using UV laser resonant two-photon ionization (R2PI). The spectra of the Watson-Crick and wobble isomers of 2PY center dot 1MU were separated using UV-UV spectral hole-burning. We identify the different isomers by combining three different diagnostic tools: (1) Selective methylation of the uracil N3-H group, which allows formation of the sugar-edge isomer only, and methylation of the NI-H group, which leads to formation of the Watson-Crick and wobble isomers. (2) The experimental S-1 <- S-0 origins exhibit large spectral blue shifts relative to the 2PY monomer. Ab initio CIS calculations of the spectral shifts of the different hydrogen-bonded dimers show a linear correlation with experiment. This correlation allows us to identify the R2PI spectra of the weakly populated Watson-Crick and wobble isomers of both 2PY center dot U and 2PY center dot T. (3) PW91 density functional calculation of the ground-state binding and dissociation energies De and Do are in agreement with the assignment of the dominant hydrogen bond isomers of 2PY center dot U, 2PY center dot 3MU and 2PY center dot T as the sugar-edge form. For 2PY center dot U, 2PY center dot T and 2PY center dot 1MU the measured wobble: Watson-Crick: sugar-edge isomer ratios are in good agreement with the calculated ratios, based on the ab initio dissociation energies and gas-phase statistical mechanics. The Watson-Crick and wobble isomers are thereby determined to be several kcal/mol less strongly bound than the sugar-edge isomers. The 36 observed intermolecular frequencies of the nine different H-bonded isomers give detailed insight into the intermolecular force field.