In previous vibrational researches, we have studied the behaviour of the peptide group in various environments. Now, infrared and Raman spectra of crystalline diketopiperazine and Raman spectra of its saturated aqueous and aqueous acid (in 6 and 12 N HCl) solutions are reported and discussed. Raman spectra of this cyclic dipeptide are not reported in the literature. A substantial modification is observed in the Raman spectra going from crystalline form to solution, and it is attributed to intermolecular hydrogen bond perturbations. Particularly, $\nu CO$ band intensity in the spectra of aqueous acid solutions decreases as acid concentration increases. At the same time, at a higher wave number, a new component appears, which intensifies as solution acidity increases. Such a new band is attributed to stretching vibration $\nu CO$ of protonated diketopiperazine molecules. The result agrees with previously discussed work about protonated acetamide.