The infrared spectra of (NH₄)₂M″(SO₄)₂.6H₂O has been analysed in the region 4000–250 cm⁻¹. The dynamics of each crystal has been discussed in terms of 234 phonon modes, including 3 acoustical ones, using the unit cell approximation. The ambiguity in the assignments of the bands in the region 900–500 cm⁻¹ has been removed by assigning the bands in this region to the libratory modes of H₂O molecules. It has been concluded that the NH₄⁺ and SO₄²⁻ ions have a symmetry lower thanT_dand also the complex [M″(H₂O)₆]²⁺ has a symmetry lower than O_h. The hydrogen bonding is the strongest in the Ni-salt and the weakest in the Mg-salt.

The existence of rocking, wagging and twisting librations of different types of water molecules in tutton salts has been established from their ir spectra. These bands have been identified, following the intensity criterion of Miyazawa. The present assignments indicate that the bands due to libratory motion of H₂O III in NH₄Mn-, KNi-, KCo and KMn- and of H₂O I in KMn- and KCu- tutton salts are out-of-plane H-bonded. The strength of the hydrogen bonding is the weakest in the (NH₄)₂Fe(SO₄)₂.6H₂O.