The 2-pyridone.2-hydroxypyridine mixed dimer, 2PY.2HP, is linked by antiparallel N-H . . .N and O-H . . .O hydrogen bonds, closely analogous to tautomeric hydrogen-bonded DNA base pairs such as isoguanine.uracil. Mass-selected S-1<--S-0 vibronic spectra of supersonic jet-cooled 2PY.2HP and three N-D or O-D deuterated isotopomers were measured by two-color resonant two-photon ionization and UV/UV-holeburning techniques. S-1-->S-0 fluorescence spectra show intense hydrogen bond stretching vibrations nu (6)' = 158 cm(-1), observed for all four isotopomers and the weak nu (5)' "shear" vibration at 116 cm(-1). The "slanting" intermolecular vibration nu (4)' = 89 cm(-1) was tentatively identified via the 2 nu (4)' overtone. The observation of 2-pyridone intramolecular overtone and combination transitions for two out-of-plane modes indicate slight nonplanarity of the 2-pyridone moiety in the S-1 state. Ab initio calculations using Hartree-Fock, configuration interaction among single excitations (CIS), and B3LYP density functional methods were employed to calculate ground- and excited-state structures, rotational constants, harmonic intermolecular and intramolecular vibrational frequencies, binding energies D-e, and dissociation energies D-0. The B3LYP/6-311++(d,p) nu (4)' and nu (6)' frequencies are in very good agreement with the experimental ground-state intermolecular frequencies. Electronic excitation of the 2-pyridone moiety in 2PY.2HP leads to a decrease of dissociation energy by 2.36 kcal/mol and a concurrent increase of H bond lengths by approximate to0.08 Angstrom, in qualitative agreement with the self-consistent field (SCF) and CIS ab initio calculations.