The crystal structure of the superionic phase of mixed trirubidium-ammonium hydrogen biselenate, [Rb0.57(NH4)0.43]3H(SeO4)2, has been determined at T = 370 K. The compound is trigonal, space group R3¯m, a = 6.073(1) Å, c = 22.636(3) Å, V = 722.9(2) Å3, Mr = 456.34, Dx = 3.12 g · cm—3, μ= 225.5 cm—1, Z = 3, λ(CuKα) = 1.541834 Å, F(000) = 631.38, R = 0.025 for 217 unique reflections with I > 3σ(I). A two-dimensional sublattice of an acid hydrogen bond network linking the SeO4 tetrahedra was found to be dynamically disordered. Despite the low occupancies of the hydrogen atoms in their crystallographic positions all hydrogen atoms are clearly observed on corresponding electron density maps. This “anomalous” display of the dynamically disordered hydrogen atoms allows the mechanism of the superprotonic transport in M3H(AO4)2 compounds to be elucidated in detail. Dynamic twinning has been observed in our study.