The strong intramolecular hydrogen bonding in 9-hydroxyphenalen-1-one (1) has been investigated by means of quantum chemical calculations and vibrational spectroscopy. Both ab initio molecular orbital (MP2) and density functional theory (B3-LYP, B3-P86) calculations predict a double minimum potential energy surface with a low (ca. 10 kJ/mol) barrier. The hydrogen-bonding energy, estimated by comparison with the non-hydrogen-bonded anti conformer, is a. 60 kJ/mol. Our comparative study of the three theoretical levels revealed the very good performance of the B3-LYP density functional in conjunction with a diffuse polarized valence triple-zeta basis set, while the B3-P86 functional tends to overestimate considerably the strong hydrogen-bonding interaction. Based on a joint analysis of the energetics and molecular geometry, the interaction in 1 can be classified as a border case between traditional and short-strong hydrogen bonds. The charge distribution refers to a strong ionic character of the hydrogen-bonding interaction. The vibrational properties of the molecule have been investigated by a combined experimental (FT-LR, FT-Raman) and theoretical analysis. The deficiencies of the computed harmonic force field were corrected by the scaled quantum mechanical (SQM) method of Pulay et al. As a result of our SQM analysis, 45 from a total of 63 fundamentals of the molecule were assigned with an rms deviation of 6.9 cm(-1) between the experimental and scaled frequencies. The most characteristic effect of hydrogen bonding on the vibrational properties of 1 is the enhanced mixing of the CO and OH vibrations with each other and with the skeletal modes.