Abstract The temperature and frequency dependent complex impedance of (NH 4) 3H(SO 4) 2 crystals was measured. The measurements have been performed in the temperature range from 27 to 220°C along the c ∗-axis of the room temperature phase. AC frequencies between 100 Hz and 5 MHz were applied. The crystals investigated in this work revealed two structural phase transitions above room temperature. The crystal becomes a fast ionic conductor in both the high temperature phases. The dielectric properties of the sample crystal in different phases were analyzed. The behavior of protonic conduction was explained by means of a power law σ′( ω)= σ(0)+ Bω s , where ω is the frequency of the applied AC electric field. The exponent s, decreases from 0.7 to 0.3 with increasing temperature for the room temperature phase but becomes almost constant (≤0.1) in the fast ionic phases. Both, the electrical conductivity and relaxation appear to be thermally activated.