Imines or Schiff bases formed through the interaction of reducing sugars with amino acids are known to play a critical role not only in initiating the Maillard reaction but also in its propagation through isomerization reactions initiated by the intermediate oxazolidin-5-one. FTIR spectroscopic evidence for the formation of this intermediate in a phenylalanine-glycolaldehyde model system was provided by taking advantage of a strong absorption band centered at 1778 cm(-1). The identity of this peak was confirmed by observing a shift to 1736 cm(-1) when [(13)C-1]phenylalanine was used. The intensity of this peak decreased over time with concomitant increase of two bands in the carbonyl absorption region, one centered at 1730 and the other at 1720 cm(-1). The former band was shifted to 1685 cm(-1), while the band at 1720 remained unchanged when [(13)C-1]phenylalanine was used. The simultaneous formation of a carboxylic acid and a carbonyl band is consistent with the formation of an Amadori rearrangement product. Furthermore, time-dependent analysis of the formation and decomposition of the oxazolidin-5-one intermediate suggests that it is an important precursor of the Amadori rearrangement product. In addition, through the use of appropriate model systems, [(15)N]phenylalanine and second-derivative spectral analysis, evidence was also provided for the formation of decarboxylated imines at 80 degrees C.