[en] The phase transitions and proton dynamics of Cs₅H₃(SO₄)₄.0.5H₂O single crystals were studied by measuring the NMR line shape, the spin-lattice relaxation time, T₁, and the spin-spin relaxation time, T₂, of the ¹H and ¹³³Cs nuclei. The 'acid' protons and the 'water' protons in Cs₅H₃(SO₄)₄.0.5H₂O were distinguished. The loss of water protons was observed above T_C1, whereas the content of water protons was found to recover above T_C2. Therefore, the water protons play a special role in the stability of the superprotonic phase at high temperatures. The mechanism of fast proton conduction was found to consist of hydrogen-bond proton transfer involving the breakage of the weak part of the hydrogen bond and the formation of a new hydrogen bond. Thus, these structural phase transitions probably involve significant reorientation of the SO₄ tetrahedra and dynamical disorder of the hydrogen bonds between them. - Graphical abstract: The phase transitions and proton dynamics of Cs₅H₃(SO₄)₄.0.5H₂O were studied by the spin-lattice relaxation time and the spin-spin relaxation time of the ¹H and ¹³³Cs nuclei. The 'acid' and the 'water' protons were distinguished. Our NMR study has provided new information about the roles of acid protons, water protons, and cesium nuclei in Cs₅H₃(SO₄)₄.0.5H₂O single crystals. Display Omitted Fig. Temperature dependences of T₁ and T₂ for acid and water protons in a Cs₅H₃(SO₄)₄.H₂O single crystal (·: T₁ and □: T₂ for acid proton, o: T₁ and □: T₂ for water proton)