The vibrational frequencies of N-acetyl-L-alanine (NAAL), its potassium salt (NAALK) and its. free anionic form (NAAL(-)) are calculated using density functional theory (B3LYP) combined with the 6311 + + G(d,p) basis set. The experimental Raman spectrum of solid NAALK and the scaling factors for calculated values are discussed as well. The three species are characterized by intramolecular NH...O hydrogen bonds leading to the formation of a five-membered ring. As indicated by the intramolecular (N)H...O distances and by the nu(NH) frequencies, the strength of the intramolecular hydrogen bond is ordered as follows: NAAL(-) < NAALK < NAAL(-). Owing to their difference in the coupling with other vibrational modes, the in-plane and out-of-plane vibrations do not reflect the strength of the hydrogen bond.