The phase transition between the two anhydrous modifications of NaHSO 4 (α and β) was studied using Raman spectroscopy and differential scanning calorimetry. These measurements indicate that β-NaHSO 4 is a metastable phase and readily undergoes phase transition to thermodynamically stable α-NaHSO 4 with an exothermic enthalpy change of 3.5kJ/mol. Both thermal (temperatures >434K) and chemical (exposure to H 2 O) pathways were identified for this transition. The transition is irreversible, and α-NaHSO 4 is an intermediate phase between β-NaHSO 4 and NaHSO 4 ·H 2 O. The possible mechanism of the phase transition is discussed.