Three modifications of the sodium hydrogen sulphate (NaHSO4·H2O, A-NaHSO4, B-NaHSO4) have been prepared. Their vibrational spectra have been measured. The results are discussed in relation to the structures of crystals. The good agreement between the crystal structure, selection rules and vibrational spectra is observed for NaHSO4·H2O crystal. However, the careful analysis of the infrared spectra of both anhydrous modifications (A-NaHSO4, B-NaHSO4) show that neither of them correspond to β-NaHSO4form. On the other hand, the infrared spectra of A-NaHSO4and B-NaHSO4modifications show that each of them contain some amount of the α-NaHSO4phase.