Paraelectric to superionic phase transition of CsH2PO4 was studied by means of X-ray diffraction and solid-state NMR. H-1 NMR powder spectrum showed the characteristic high field shift (8.0 ppm relative to H2O) as is expected for the proton forming a strong hydrogen bonding. The temperature dependence of the H-1 and P-31 NMR suggests a reorientational jump of the H2PO4- anion above 420 K. The reorientational jump of the H2PO4- anion was also confirmed by the P-31 NMR using a single crystal and the activation energy was estimated 4 to be 0.74(1) eV by the spectrum simulation. Above the superionic phase transition at 502 K, a dynamically disordered PO4 unit was confirmed by the Rietveld analysis and also by the isotropic sharp spectra of the H-1, P-31 NMR. These observations suggest that the reorientation of the H2PO4- anion plays air important role for the proton conduction even in the paraelectric phase. (C) 2004 Elsevier B.V. All rights reserved.