Oxazole ring occurs in numerous natural peptides, but conformational properties of the amino acid residue containing the oxazole ring in place of the C-terminal amide bond are poorly recognized. A series of model compounds constituted by the oxazole-amino acids occurring in nature, that is, oxazole-alanine (L-Ala-Ozl), oxazole-dehydroalanine (Delta Ala-Ozl), and oxazole-dehydrobutyrine ((Z)-Delta Abu-Ozl), was investigated using theoretical calculations supported by FTIR and NMR spectra and single-crystal X-ray diffraction. It was found that the main feature of the studied oxazole-amino acids is the stable conformation beta 2 with the torsion angles phi and psi of -150 degrees, -10 degrees for L-Ala-Ozl, -180 degrees, 0 degrees for Delta Ala-Ozl, and -120 degrees, -0 degrees for (Z)-Delta Abu-Ozl, respectively. The conformation beta 2 is stabilized by the intramolecular N-H center dot center dot center dot N hydrogen bond and predominates in the low polar environment. In the case of the oxazole-dehydroamino acids, the pi-electron conjugation that is spread on the oxazole ring and C-a=C-beta double bond is an additional stabilizing interaction. The tendency to adopt the conformation beta 2 clearly decreases with increasing the polarity of environment, but still the oxazole-dehydroamino acids are considered to be more rigid and resistant to conformational changes.