Fast proton-conductive materials containing no water are promising in application to solid electrolytes in fuel cells. Some inorganic solid acid salts show high proton conductivity. We have studied proton dynamics in CsHSO₄ by means of ¹H solid-state NMR. The proton mean residence times are determined, which can well explain the macroscopic electric conductivity, as shown in Fig. 1. Proton transfer between two neighboring SO₄ tetrahedra takes place much more easily than the reorientation of the SO₄ tetrahedra in both the room-temperature phase and the high-temperature phase. Thus, the SO₄ reorientation limits the rate of the proton transport.