In order to establish parameters to identify imide derivatives formed during in vitro aging of aspartyl-alanyl-containing proteins, a dipeptide and a tetrapeptide containing this sequence were acidified and heated in vacuo. The formation of succinimide derivatives could be confirmed by FTIR and Raman spectroscopy. Hereto, earlier assignments of succinimide vibrations had to be revised. FTIR absorbance spectra of the succinimide, derived from H-Asp-Ala-OH, in the solid state give five bands between 1700 and 1800 cm-1. In this case, owing to Fermi resonance, the anti-symmetric imide carbonyl stretching vibration gives rise to an apparent doublet, centered around 1715 cm-1. The symmetric stretching mode is found at 1793 cm-1. The other bands are assigned to carboxylic acid stretching modes (1728 cm-1; COOH dimer and 1751 cm-1: COOH monomer). Fermi resonance does not occur in succinimide derivatives occurring in larger peptides. As a consequence, the imide bands of the succinimide, generated from H-Val-Asp-Ala-Gly-OH, are observed at 1716 and 1791 cm-1 (anti-symmetric and symmetric stretching modes, respectively).