In order to investigate the proton dynamics and the mechanism of the relatively high-electrical conductivity observed even below Tc in Rb3H(SeO4)(2), we have measured the H-1 spin-lattice relaxation rate. From the measurement of the 1 H spin-lattice relaxation rate T-1(-1), it is found that the temperature dependence of T-1(-1) takes a peak at around 345 K. The analysis of the temperature dependence of T-1(-1) shows that T-1(-1) in Rb3H(SeO4)(2) is described well by the dipole-dipole relaxation between proton and Rb nuclei. Moreover, it was also found that not only the proton dynamics described by a hopping motion of proton between two equivalent sites in the hydrogen bond but also a new process of proton transfer appears above around 338 K. This new process is a hopping motion of proton accompanied by the breaking and rearrangement of hydrogen bond. From these results it is deduced that the increase of the electrical conductivity observed even below T. is caused by the proton transfer between possible stable sites accompanied by the breaking and rearrangement of hydrogen bond, and that electrical conductivity in Rb3H(SeO4)2 is dominated by protons motion even below T,. (c) 2007 Elsevier B.V All rights reserved.