Proton dynamics in (NH4)(3)H(SO4)(2) has been studied by means of H-1 solid-state NMR. The H-1 magic-angle-spinning (MAS) NMR spectra were traced at room temperature (RT) and at Larmor frequency of 400.13 MHz. H-1 static NMR spectra were measured at 200.13 MHz in the range of 135-490K. H-1 spin-lattice relaxation times, T-1, were measured at 200.13 and 19.65 MHz in the ranges of 135-490 and 153-456 K, respectively. The H-1 chemical shift for the acidic proton (14.7 ppm) indicates strong hydrogen bonds. In phase III, NH4+ reorientation takes place; one type of NH4+ ions reorients with an activation energy (E-a) of 14 kJ mol(-1) and the inverse of a frequency factor (tau(0)) of 0.85 x 10(-14) s. In phase II, a very fast local and anisotropic motion of the acidic protons takes place. NH4+ ions start to diffuse translationally, and no proton exchange is observed between NH4+ ions and the acidic protons. In phase I, both NH4+ ions and the acidic protons diffuse translationally The acidic protons diffuse with parameters of E-a = 27 kJ mol(-1) and tau(0) = 4.2 x 10(-13) s. The translational diffusion of the acidic protons is responsible for the macroscopic proton conductivity, as the NH4+ translational diffusion is slow and proton exchange between NH4+ ions and the acidic protons is negligible. (c) 2006 Elsevier B.V. All rights reserved.