Although the importance of alpha-dicarbonyl compounds as reactive intermediates in the Maillard reaction and as precursors of heterocyclic and odor-active compounds is well-established, however, the detailed origin of many alpha-dicarbonyl compounds such as 3,4-hexanedione and 1,2-butanedione still remains unknown. Using glucose and glyoxal with labeled [(13)C-1]alanine, [(13)C-2]alanine, [(13)C-3]alanine, and [(15)N]alanine, the mechanism of their formation was investigated using the label incorporation pattern of the pyrazines derived through the Strecker reaction. Taking into account the non-oxidative mechanism of pyrazine formation, the data indicated that all of the ethyl-substituted pyrazines identified in the glyoxal/alanine model system incorporated C-2' and C-3' atoms of alanine, and not that of free acetaldehyde, as the ethyl group carbon atoms. This was achieved through spiking experiments using unlabeled acetaldehyde in the presence of labeled alanine. Furthermore, the data also indicated the occurrence of a chain elongation process of sugar-derived alpha-dicarbonyl compounds assisted by alanine. On the basis of the proposed mechanism, the glyoxal interaction with alanine through a decarboxylative aldol addition reaction can lead to the formation of 1,2-butanedione with the terminal ethyl carbon atoms originating from C-2' and C-3' atoms of alanine, and the similar interaction of 1,2-butanedione with a second molecule of alanine can lead to the formation of 3,4-hexanedione with both terminal ethyl carbon atoms originating from C-2' and C-3' atoms of alanine.