Proton dynamics in Cs-3(HSO4)z(H2PO4) has been studied by means of H-1 solid-state NMR as well as thermal analyses. The thermal analysis shows an endothermic peak at 408 K, which corresponds to a superprotonic transition. Above the transition temperature a mass loss is observed in a dry atmosphere, which is easily recovered in a conventional dry atmosphere below the transition temperature. The H-1 magic-angle-spinning NMR spectra at room temperature show two peaks at 13.5 and 15.8 ppm, and a shoulder at 11.3 ppm from tetramethylsilane, demonstrating a presence of several inequivalent proton sites. Translational diffusion of protons takes place in both a room-temperature phase (RT) and a high-temperature phase (HT). In both phases reorientation of the SO4/PO4 tetrahedron limits the rate of the proton transport. The H-1 mean residence times are estimated as E-a=33 kJ mol(-1) and tau(0)=0.97 x 10(-9) s for phase RT from the second moment analysis and as E-a=20 kJ mol(-1) and tau(0)= 5.0 x 10(-12) s for phase HT from the analysis of the H-1 T-1 results. (c) 2006 Elsevier B.V. All rights reserved.