Rb 3H(SeO 4) 2 single crystal undergoes a transition from low-temperature ferroelastic phase to superionic, paraelastic one at T S≈456 K. The low-temperature monoclinic structure contains a zero-dimensional hydrogen bond network. Above T S, the reorienting HSeO 4 groups form a dynamic hydrogen bond network in the (001) plane. The presented results of proton conductivity, ferroelastic domain pattern and differential scanning calorimetry (DSC) studies in the vicinity of T S show that the transition region is stretched into ∼10 K and the transition process proceeds in steps: the changes in the ferroelastic domain pattern close to T S are accompanied by DSC anomalies. The temperature dependence of the conductivity σ [100] on heating and cooling revealed ∼2 K hysteresis of the superionic phase transition. The experimental results support the suggestion of recent papers on theoretical studies of the behaviour of protonic conductivity in M 3H(XO 4) 2 crystal group in the superionic phase transition region.