In order to understand the thermodynamic properties and structural geometry of NH₄Al(SO₄)₂·12H₂O, we studied the magic angle spinning nuclear magnetic resonance (MAS NMR) and static NMR as a function of temperature. The changes of the chemical shifts, linewidths, resonance frequencies, and spin–lattice relaxation times for ¹H, ¹⁴N, and ²⁷Al nuclei near T_d (=335 K), which is interpreted as the onset of partial thermal decomposition, are due to the structural phase transition. The changes near T_d are related to the loss of H₂O, which probably disrupts the forms of the octahedra of water molecules surrounding NH₄⁺ and Al³⁺ ions. This mechanism above T_d is related to hydrogen-bond transfer involving the breakage of the weak part of the hydrogen bond.