Proton dynamics in [(NH4)(1-x)Rb3H(SO4)(2) with x=0.54 has been studied by means of H-1 solid-state NMR. The H-1 magic-angle-spinning (M AS) NMR spectra were traced at room temperature (RT), and H-1 static NMR spectra and spin-lattice relaxation times (T-1) were measured in the range of 296-490 K. The H-1 chemical shift for the acidic proton (15.4 ppm) indicates hydrogen bonds of intermediate strength between (NH4)(3)H (SO4)(2) and Rb3H(SO4)(2). In phase II, a very fast local and anisotropic motion of the acidic protons takes place and NH4+ ions start to diffuse translationally just below the transition temperature. In phase 1, both NH4+ ions and the acidic protons diffuse translationally. The acidic protons diffuse with an activation energy of 29 kJ mol(-1) and the inverse of a frequency factor of 6.0 x 10(-13) s. No proton exchange is observed between NH4+ ions and the acidic protons in both phases. (C) 2008 Elsevier B.V. All rights reserved.