Ab initio quantum-chemical calculations were performed to investigate the interconversion barriers for the most stable glycine and α-alanine conformers. The calculations were carried out at the HF and MP2 levels of the theory with the double split valence basis set augmented with polarisation and diffuse shells on all atoms (6–3I++G**). The tow energy barriers separating some glycine and α-alanine conformers were found due to the calculations. They allow the conformers easily to interconvert to lower energy forms. The results obtained are in good agreement with the experimental studies of the glycine and α-alanine conformational structure and explain why from three glycine conformers and five α-alanine conformers predicted to have relative energies within 7 kJ/mol only two conformers of every amino acid were observed experimentally and why the third stable glycine conformcr was observed in low temperature inert gas matrices only at the temperature less than 13 K. The results presented demonstrate that structural investigations of the flexible molecules such as amino acids and oligopeptides having many-minimum potential energy surface should be based not only on the conformer relative energies but the barriers between the conformers must be taken into account.