Phase transitions and proton dynamics in [(NH4)(1-x)Rb-x](3)H(SO4)(2) (0 < x < 1) have been studied by means of differential scanning calorimetry (DSC), thermnogravimetric and differential thermal analyses (TG-DTA) and H-1 solid-state NMR. Thermal analyses demonstrate that a phase transition from a room-temperature phase (phase II) to a high-temperature phase (phase I) takes place. The transition temperature increases with x. On the other hand, the melting/decomposition temperature is almost independent of x. Thus, the temperature range of phase I become narrower with x. The H-1 MAS NMR spectra at room temperature demonstrate that the acidic protons form the stronger hydrogen bond with increase in x. This trend might correlate with the transition from phases II to I. H-1 static NMR spectra indicate that the proton motions in [(NH4)(1-x)Rb-x](3)H (SO4)(2) (0 < x < 1) are similar to those in (NH4)(3)H(SO4)(2) and Rb3H(SO4)(2), although the temperature ranges are different. In phase I, both NH4+ ions and the acidic protons diffuse translationally, which supports that phase I is superprotonic. (c) 2008 Elsevier B.V. All rights reserved.