Structural relaxation and the role of molecular and proton dynamics in the electrolyte system, (NH 4 ) 4 H 2 (SeO 4 ) 3 (TeAHSe), are studied at ambient pressure, as a function of temperature and time, by using the solidstate 1 H NMR technique. Analysis of 1 H NMR spectra collected in the temperature range from 20 to 400 K yielded information about the dynamics of protons in the ordered and disordered crystalline TeAHSe phases. It has been shown that in the low-temperature phase TeAHSe is a proton conductor. In contrast, the electric charge carriers in the superionic phase are protons and NH 4 + ions. Therefore, the electrical conductivity in the superionic phase of TeAHSe is a combination of the chemical exchange of protons, proton diffusion within hydrogen bonds, and diffusion of ammonia cations in the bulk of the crystal. We also demonstrate that the enhanced charge transport in the superionic phase of TeAHSe mainly results from the NH 4 + cation diffusion process. The unique data from the time evolution of NMR spectra taken at a constant temperature just below the phase transition provide the first direct evidence of ammonium cations involvement in the lowtemperature structure recovery (structural relaxation). We proposed a scenario of the process of structural relaxation in the TeAHSe crystal as well as the origin of the phase transformation mechanism to the superionic stage.