Proton diffusion in [(NH4)(1-x)Rb-x](3)H(SO4)(2) (0 < x < 1) has been studied by means of H-1 spin-lattice relaxation times, T-1. The relaxation times were measured at 200.13 MHz in the range of 296-490 K and at 19.65 MHz in the range of 300-470 K. In the high-temperature phase (phase I), translational diffusion of the acidic protons relaxes both the acidic protons and the ammonium protons. Spin diffusion averages the relaxation rate of the two kinds of protons, whereas proton exchange between them are slow. The spin-lattice relaxation times in phase I were analyzed theoretically, and parameters of proton diffusion were obtained. The mean residence time of the acidic protons increases with increase in x for [(NH4)(1-x)Rb-x](3)H(SO4)(2) (0 <= x <= 0.54). Rb3H(SO4)(2) does not obey this trend. The results of NMR well explain the macroscopic proton conductivity. (c) 2008 Elsevier B.V. All rights reserved.